Interstitial cystitis (IC) is a chronic inflammatory disorder of the urinary bladder of unknown aetiology and pathogenesis. The classic form (Hunner's ulcer) is characterized by high mast cell numbers in the detrusor muscle and an expansion of mucosal mast cells in the lamina propria and also in the epithelium. Such cells can be recovered in bladder washings and in the urine in single cell suspensions. We have counted the mast cells and measured the histamine in bladder washings from 16 patients with classic IC and from a control group of 15 patients with so-called early, non-ulcerative IC. The bladder washings from all patients with classic IC contained well preserved mast cells (median 2.16, range 0.5-8.6 x 10(3) cells/I) and histamine (median 14.3, range 6-66 ng/l), while only occasional mast cells and traces of histamine were found in washings from patients with non-ulcerative IC. The histamine content was strongly correlated to the number of mast cells (r = 0.87). The mean histamine content per mast cell was estimated at 7.6 +/- 0.65 (SEM) pg/cell. The high histamine content per mast cell in relation to previously published data (2.8-4.6 pg/cell) can be attributed to the mild and rapid handling of the specimens.