In a retrospective study we compared the outcome of 39 patients with interstitial cystitis to the histological findings at initial diagnostic bladder biopsy. The degree of inflammation and fibrosis and the mast cell counts were assessed on each biopsy. The prognostic relevance of the clinical features of age, duration of symptoms, frequency, nocturia, pain and bladder capacity was assessed. The study showed no statistical correlation between the severity of histological findings at diagnosis and the eventual outcome of the disease. Over 50% of patients with severe histological abnormalities responded to conservative treatment. Although the majority of patients with mild pathological changes responded to conservative treatment, some did require surgical intervention. The clinical features of pain, nocturia and bladder capacity showed significant differences between the 2 patient groups. However, the former 2 features are subject to many variables and the latter probably has too wide a range to be useful as a prognostic indicator. Only the response of patients to conservative treatment will indicate those who are not being helped and who may eventually require surgical treatment.