Almost all the patients (95 women and 8 men) with interstitial cystitis, i.e. autoimmune cystitis, in this area with a population of about 970,000 were probably traced. The prevalence of the disease in the female members of this population was 18.1 cases per 100,000 women of all ages. The joint prevalence of both sexes together was 10.6 cases per 100,000. The annual incidence of new female cases was 1.2 per 100,000 women. The disease is not rare when mild and moderately severe cases are also diagnosed. Severe cases account for only a tenth of all cases. Only about a tenth of the patients are men and most of them had the mild form of the disease. It may begin at any age. The incidence has possibly been rising during the last 10 years. Interstitial cystitis does not as a rule progress continuously but reaches its final stage rapidly, then usually remains in the same category. With this prevalence and incidence every urologist in this area can expect to see at least one case a year.