We evaluated the relation between occupational exposure to engine exhaust fumes and cancer risk among members of a large prepaid health plan who reported on exposure during a routine health examination (n = 160,230). Exposure in the past year was associated with an elevated risk of cancer of the thyroid (relative risk (RR) = 1.99; 95% confidence interval (CI), 1.01-3.92), female breast (RR = 1.53; CI, 1.00-2.33), nonbrain nervous system (RR = 2.26; CI, 1.09-4.67), and lip/tongue (RR = 1.82; CI, 1.09-3.04), and a decreased risk of melanoma (RR = 0.50; CI, 0.27-0.90). However, another measure of exposure that included both exposure prior to 1 year and exposure in the past year was associated only with cancer of the lip/tongue (RR = 1.82; CI, 1.02-3.32). No association was observed for lung, bladder, or larynx cancer or multiple myeloma. Analyses limited to men, or stratified by time since health examination, did not distinguish other effects. Self-reported occupational exposure to engine exhaust fumes was not convincingly associated with most cancers in this cohort.