We observed nine cases of transitional-cell carcinoma of the urinary bladder among patients who had had long-term treatment of other cancers with cyclophosphamide. Seven of the bladder carcinomas occurred within a cohort of 471 patients treated for non-Hodgkin's lymphomas. In this cohort the relative risk of bladder cancer was 6.8 (95 percent confidence interval, 3.2 to 14.2). The cumulative risk (mean +/- SE) was 3.5 +/- 1.8 percent 8 years after the start of treatment with cyclophosphamide and 10.7 +/- 4.9 percent after 12 years. Three of the nine patients were 50 years of age or younger; seven died with progressive bladder cancer. Subsequently, an additional patient had acute nonlymphocytic leukemia. Hemorrhagic cystitis was observed in 33 patients (cumulative risk, 11.8 +/- 2.1 percent after five years). Development of carcinoma of the urinary bladder was not related to previous hemorrhagic cystitis. The results caution against long-term treatment with cyclophosphamide for diseases with a favorable prognosis.