Before Joni Mitchell consulted a mystic healer on the matter of the bleeding lesion in her throat, she asked some conventionally qualified, AMA doctors about it and they informed her it was what Sammy Davis Jr had. The late Sammy Davis Jr. They told her it was caused by smoking and gave her five years.

Five years later when Joni Mitchell tells this story, we are approximately half way through our interview and she is approximately half way through a packet of Marlboro Lights. Mitchell chain-smokes, and it's one of the smoothest chains you've ever seen. As her left hand stubs one out, her right is already loosening the next from the packet. She's been smoking since she was nine - maybe literally - and she wasn't about to quit simply because her voice packed up during a video shoot and doctors were showing her fibre optic shots of an open wound on her larynx.

"I grasped that smoking was an irritant," she says, "I didn't believe it was a cause." So (and here the story takes a mildly Californian twist) she sought a second opinion from a Hawaiian mystic called Oleta.

"She's a two-powered mystic. With you fully dressed and lying on a table, she can see into your body by going wall-eyed, seeing light and shadow through your clothes. I said, 'Look in my throat, Oleta. Do you see death there?' 'No,' she said. This sounds so crackpot: she sent me water. The water was electrically charged and commanded to sluice and slowly restore. She fixed me.

"It's healed up. My voice is fragile, but I do believe I'm singing better than ever in my life. I'm on the brink of being a great singer. I've lost my high end but I don't miss it - you don't need it. I had three and a half octaves, all of that stratospheric stuff was just trying to impress. Billie Holiday had seven notes. And what she did with it."

We meet in the office of Mitchell's manager, Peter Asher, in Los Angeles. Mitchell drives down from her house in the hills, parks an impressive black Land Cruiser by the pavement opposite and runs across the road. She is wearing blue-striped trousers and a belted jacket. She has leather sandals on bare feet. In the way she moves and the way she talks and especially in the way she laughs (a childish hee-hee-hee) she does not appear to be 50.

Mitchell grew up in Fort Mcleod, Alberta, Canada and had a childhood marred by illness. At Christmas in 1952, the year before the vaccination came out, Mitchell she developed polio. She was nine and she says it made her prematurely adult and stubborn.

"The polio ward is a really depressing place and you hear the whining of the iron lungs, a bunch of them going away, and you're just praying that you don't go into one. The disease only rampages for two weeks and then you're left with the disaster. I was unable to walk or stand. I was train-wrecked. My spine looked like the freeway after an earthquake. An adult male doctor could put two fists under the arch of my back. My mum put up a little Christmas tree in my room and I remember saying to that tree, 'I am not a cripple.' They would come with cauldrons of hot flannel rags and pin them all over you - the heat was meant to do something to the muscles. In a very short space of time, I unfurled. They sent me home in a wheelchair, but I refused to use it. Polio survivors - Neil Young is another one - are a really stubborn bunch of people."

She shows off a copy of the picture she has painted for the sleeve of her new album, a spoof of a Van Gogh self-portrait, with Mitchell as Van Gogh, looking a bit grumpy with her ear in a wad of bandage. It's a neat joke at her own expense: Mitchell the tortured artist. Turbulent Indigo is her first album for three and a half years. It is strong and angular and features dextrous ensemble playing but keeps falling back to Mitchell and her guitar alone - the point at which she came in. The album follows - and squashes - a number of alarming rumours about the extent of her torture; rumours that she had given herself entirely to her painting; rumours that she was hardened for good against touring; rumours that she was suffering from cancer and incapable of touring.

In fact, much of the delay was contractual business. She has shifted record company, moving from Geffen to the Reprise label, trying to strike a deal which compensates for ways she feels she has been poorly done by in the past and delaying her album hugely in the process. It was ready to come out more than a year ago. Since then, she says, "I've been up to my neck in alligators".

Also she separated recently from Larry Klein, the bassist and producer she married in 1982. "He was very young when we married. He was 25 and I was 38. We are really good friends. I love this man, this is a good man. I've chosen good men in my life to be associated with. But in 12 years we probably spent more time together than people who are married for 60 years. Klein and I lived in each other's pocket. I instigated the change, for both our own good.'' She has a new relationship now. ''A writer in residence in Saskatchewan. My mother introduced me to him. She wanted me to help him. I'm not sure she wanted me to... Now I'm a pretty happy camper, subject to useful depression. No need to call in the Prozac or anything."

But without worry, there would be no Joni Mitchell music. She has worried in song about big business, the ecology, urban development, relationships, whales. ''It's harder to write out of joy. You can, after the fact, but worrying is a really good constructive meditation for songwriting. Almost anything you're worrying about, there's a long line behind you."

Nowadays she worries about not being able to afford the perpetual maintenance on her house. ("Two years ago, it cost me to paint the trim the same amount it cost me to buy my first home in LA.'') And then there's the upkeep on her country retreat in Canada. "The problems of the rich - those don't make good songs somehow, although someday I'm gonna tackle that, as a theme. Shakespeare would have."

In many ways, Mitchell's key project has been the musicalisation of worry and doubt: as a folk artist with a guitar and later at the front of abstract and hugely supple bands. She was not classically taught. She describes herself as ''a sophisticated intuitive" who once noticed Buffy Saint-Marie de-tuning her guitar and began to explore variant tunings herself, plotting her way to those carefully widened chords which characterise her sound. ''Most people like their optimism major and their tragedy minor. Well, my life's not that simple. Within a chord that depicts optimism there's a thread of doubt."

In the church choir as she grew up, she sang descant, drawn to it, she says, "because of all the lines going through a hymn, the descant seemed to have the most movement in it.'' This explains part of what is distinctive about the melodies she went on to write. They sound like a descant for a melody which the song isn't supplying you with. The word often tossed in the direction of this work is jazz, but as she points out, her melodies have been known to alienate jazz players too; Victor Feldman, who played vibes on ''Moon at the Window'' told her, frankly, that he "hated the harmony". Her melodic invention seems to occupy its own turf.

"I'm going into my fifties as a female songwriter, and there aren't a lot of role models for this. But I'm a pioneering spirit. I come from pioneering stock. I realise we were the generation who said all people over 30 should be shot, but I'm not going to put my hair in a bun and wear black. I need to create a vital middle age.

"So I'm putting my pitch. A vote for me is a vote for honest Joan. If you don't buy my records, I'll probably be dumped in a couple of years."