Specific clinical criteria have been used to diagnose interstitial cystitis in 32 patients. The distribution of mast cells in biopsies of the bladder wall from these patients has been compared with that of similar cells in control specimens. A highly significant increase (P<0.001) in the number of mast cells within the detrusor muscle bundles has been demonstrated in 27 of these patients. This mast cell infiltration is widespread throughout the detrusor muscle and is not confined to the ulcerative lesions seen cystoscopically. Histological estimation of mast cells is of value, therefore, in establishing the diagnosis of interstitial cystitis, and may be particularly helpful in equivocal cases. The relationship between mast cell infiltration of the detrusor muscle bundles and the aetiology of the disease remains to be determined.