The antitumoral effects of intravesical bacillus Calmette-Guerin against recurrent superficial urothelial bladder cancer seem to be linked to immunological effector mechanisms. To characterize further the local cellular response in the bladder wall of patients receiving intravesical bacillus Calmette-Guerin bladder biopsies were examined before and immediately after a 6-week course of bacillus Calmette-Guerin and for up to 1 year at 3-month intervals thereafter. The results showed a marked infiltration after bacillus Calmette-Guerin therapy predominantly of the suburothelial tissue with mononuclear cells, which expressed activation markers and accumulated to follicle-like granulomatous structures. With regard to T lymphocyte subsets the ratio of T helper-to-T suppressor cells in the bladder wall increased from 0.5 to nearly 2. This phenomenon persisted even in biopsies 1 year after initial treatment. In some probes activated mononuclear cells invaded the urothelium, emanating from suburothelial granulomas. It is tempting to assume that these changes within the local immunocompetent cells are related to the therapeutic effect of bacillus Calmette-Guerin against bladder cancer.