The relationship between employment in occupations with potential exposure to motor exhaust and bladder cancer risk was examined based on interviews conducted with 1909 white male bladder cancer patients and 3569 population controls during the National Bladder Cancer Study, a population-based, case-control study conducted in ten areas of the United States. Our findings indicated that males usually employed as truck drivers or deliverymen have a statistically significant, 50% increase in risk of bladder cancer. Overall, a statistically significant trend in risk with increasing duration of truck driving was observed. This trend was particularly consistent for drivers first employed at least 50 years prior to diagnosis. Of these, truck drivers employed 25 years or more experienced a 120% increase in risk. Elevations in risk were also suggested for taxicab and bus drivers. These findings, coupled with experimental evidence of the mutagenicity and possible carcinogenicity of motor exhaust emission particulates, suggest a role for motor exhaust exposure in human bladder carcinogenesis.