The prostates of 36 patients who were treated with intravesical bacillus Calmette-Guerin were evaluated by digital rectal examination and transrectal ultrasonography. When abnormal palpatory and/or ultrasonographic findings were detected, core needle biopsies from the suspicious areas were performed. Of the 36 patients 20 underwent biopsies of the prostate. Pathological findings revealed typical granulomas in 8 patients (3 caseating and 5 noncaseating multifocal granulomas). Nonspecific chronic prostatitis was noted in 4 patients and benign prostatic hyperplasia was noted in 8. The number of bacillus Calmette-Guerin instillations ranged from 6 to 19. The interval from initiation of therapy to biopsy ranged from 1.5 to 14.5 months. Caseating granulomas were found during the early course of bacillus Calmette-Guerin instillations (1.5 to 3.0 months), whereas noncaseating granulomas were detected at later stages (4 to 14.5 months). These findings present a high incidence of granuloma formation in patients treated with intravesical bacillus Calmette-Guerin. The duration of therapy is a determinant factor in the induction of granuloma type.