The effects of 5% and 50% partial cystectomies on bladder tumorigenesis initiated with N-butyl-N-(4-hydroxybutyl)nitrosamine(BBN) were investigated in Wistar rats by examining the histological findings and cell proliferative activity. The incorporation of bromodeoxyuridine (BrdUrd) into the DNA synthesis phase was determined by an in-vitro labeling technique. After eight weeks of treatment with drinking water containing 0.05% BBN, 5% or 50% partial cystectomy was performed at the end of week 16, and the resected bladder was sutured with dexon or silk. There was no difference in the incidence of papillary or nodular (PN) hyperplasia between the control and partial cystectomy groups. However, the incidence of cancer in the group given partial cystectomy was much higher than that in the control. All the cancers in the control group were grade-1 superficial tumors, whereas grade-2 or invasive tumor was observed in six of 40 animals in the partial cystectomy group. The 31.0% labeling index of cancer in the partial cystectomy group was greater than the 24.1% in the control group. There was also a significant difference in the number of BrdUrd-labeled cells in PN hyperplasia between the control and partial cystectomy groups. These findings indicate that partial cystectomy enhances BBN-initiated bladder carcinogenesis and the increase in DNA synthesis found in PN hyperplasia and cancer may be associated with the induction of bladder tumors.