In nearly all populations studied, the risk of bladder cancer is two to four times as great in men as in women. We estimated what the gender-specific incidence rates would be in the absence of exposure to known carcinogenic factors. The data used were obtained from interviews with 2,806 white individuals with bladder cancer and 5,258 white controls in the National Bladder Cancer Study and from incidence data for 1978 from the National Cancer Institute Surveillance, Epidemiology, and End Results Program. The total age-adjusted incidence of bladder cancer was 27.5 cases per 100,000 person-years for men and 7.0 for women, yielding a ratio of 3.9. Even in the absence of exposure to cigarettes, occupational hazards, or urinary tract infection, the gender-related risk persisted; the incidence of bladder cancer was 11.0 in men and 4.1 in women, yielding a ratio of 2.7. Possible explanations for the excessive risk in men include environmental and dietary exposures not yet identified and innate sexual characteristics such as anatomic differences, urination habits, or hormonal factors.