We assessed the risk of occurrence of cancer associated with exposure to metronidazole in the 771 female residents of Rochester, Minnesota, who were treated with metronidazole for vaginal trichomoniasis during the period 1960 through 1969 and were followed up for a total of 12,628 person-years. Standardized morbidity and mortality ratios were determined by using an expected number calculated by applying age-specific incidence rates from Rochester studies and Cancer Surveillance, Epidemiology, and End-Results Reporting (SEER) data to the person-years of follow-up. The overall standardized morbidity ratios for cancer at all sites were 1.4 (Rochester, 1978 through 1983), 1.5 (SEER data for Iowa, 1978 through 1981), and 1.2 (SEER data for Connecticut, 1978 through 1981). By site of the cancers, the standardized morbidity ratios greater than unity were those for malignant lesions of the lung, breast, thyroid, bladder, brain, kidney, nasopharynx, and oral cavity, as well as for multiple myeloma and malignant melanoma; however, the only significantly elevated standardized morbidity ratio was that for bronchogenic carcinoma. After adjustment for smoking status, the standardized morbidity ratio for bronchogenic cancer was 2.5 (95% confidence interval of 1.3 to 4.4). The standardized mortality ratio for cancer at all sites was 1.4 (95% confidence interval of 0.9 to 2.2). The analysis of these data suggests no significant increase in cancer-related morbidity or mortality for women exposed to metronidazole for treatment of vaginal trichomoniasis.