'Dear Diary, what a God-awful day I've had... the stress of it all groan, moan, gripe, grizzle...' If this is how your daily journal reads, you could be doing yourself a power of good - especially if you have a chronic illness like asthma or arthritis.

It is no news that 'disclosing' - commonly known as pouring your heart out - can help you cope with stressful or distressing situations. It's the basis of every form of counselling from psychotherapy to old-fashioned gossip. But according to researchers from the University of Dakota's Center for the Advancement of Health, putting pen to paper, even very briefly, does more than ease psychological distress. It actually improves physical health. Indeed, for some conditions, it appears doctors would be well advised to prescribe writing at the same time as writing a prescription.

The evidence comes from a fascinating study published in today's issue of the Journal of the American Medical Association. In it, 48 people with asthma and 35 people with arthritis were asked to write about the most stressful experience in their lives for 20 minutes on three consecutive days. At the same time a control group of 22 asthma patients and 21 arthritis patients were asked simply to write about their plans for the day. All the patients carried on with their regular medications.

Four months later, the research team found almost half those who wrote about stressful events showed a clinical improvement in their symptoms, compared to 24 per cent in the control groups. In the asthmatic group, those who wrote out their innermost feelings experienced almost a 20 per cent improvement in their lung function - a measure that reflects ease of breathing - compared to no change in those who just planned their day. For the arthritis suffers, emotional writing reduced the severity of their disease by almost a third, with again no change in the control group.

Researcher Dr Joshua Smyth cautions that writing should not replace prescribed treatment and that it's not known whether the impact of the writing task carries on beyond four months. Nevertheless, health benefits seem a good return for a hour's paperwork.

But how is it that simply writing about a stressful event - and not even one necessarily related to your illness - can be effective medicine? According to Dr Geoffrey Lowe, a health psychologist at the University of Hull who is investigating, among other things, the benefits of reading and writing poetry on health, the answer is far from clear but has something to do with a link between psychological release and the immune system.

"There is now considerable evidence that self disclosure boosts the immune system. People who write - or speak - about their troubles and traumas seem to have a better immune system." The advantage of writing, over the classic psychotherapy approach of talking, says Dr Lowe, is that not only is there no fear of judgement or exposure, but the cost of a Biro and a pad of £4 is cheaper than an hour's treatment on a therapist's couch.

It's tempting to think that reliving emotionally-taxing experiences would drain immune capacity rather than boost it, but a test carried out by the University of Miami shows the exact opposite. Students in Florida who were healthy but carrying the Epstein Barr virus (EBV) which causes glandular fever, were asked to write or talk about stressful events once a week. After three weeks, those who talked or wrote about stress had significantly lower levels of EBV antibodies in their blood - showing better control by the immune system of the latent virus - than a control group who wrote and talked about trivial events.

But a burst of words of paper doesn't only help the unwell, say the Dakota researchers. "Previous studies show that healthy individuals who perform similar brief writing tasks, those we set patients with asthma and arthritis report fewer medical symptoms, greater well-being and less use of health care services," says Dr Smyth.

This was certainly the case with 122 students at an American university who, at the start of the college year, were asked to write either about their thoughts and feelings on entering college and formulate plans on how to cope with their new life, or to write about trivial topics in three, weekly writing sessions. Those who wrote serious stuff had fewer illness-related clinic visits in the following month, and over time achieved higher grade point averages. Sick or well, it doesn't hurt to pick up a pen and emote for your health.