Summary— Interstitial cystitis (IC) is a painful condition in the urinary bladder of unknown aetiology and pathogenesis. To assess the contribution of the nervous system to IC a biopsy was taken from 6 patients with IC and from a control group of 6 patients with non-ulcerative IC. Mast cells were counted and histamine measured in bladder washings from all patients. There were significantly more nerve fibres within the suburothelium and detrusor muscle in chronic IC than there were in non-ulcerative IC. The bladder washings from all patients with IC contained mast cells and histamine, while only occasional mast cells and traces of histamine were found in washings from patients with non-ulcerative IC. There was a good correlation between the number of nerve fibres and number of mast cells as well as between the number of nerve fibres and the amount of histamine.