BACKGROUND The study of a possible relationship between vasectomy and prostate cancer has yielded mixed results. Data from developing countries are limited. STUDY DESIGN We conducted a hospital-based case-control study in China, Nepal and the Republic of Korea to evaluate the risk of prostate cancer after vasectomy. RESULTS Prostate cancer in 294 cases (confirmed by independent pathologists) and 879 matched controls were included. The odds ratio of prostate cancer in men with a history of vasectomy was 1.21 [95% confidence interval (95% CI)=0.79, 1.87]. No significant trend was observed in risk by time since vasectomy or age at vasectomy. The odds ratio for localized disease was 1.02 (95% CI=0.53, 1.95); the odds ratio for later stages was 1.41 (95% CI=0.78, 2.53). No confounding factor was identified. The study illustrated differential misclassification of disease by vasectomy status; reference pathologists determined that 28% of men with a history of vasectomy, compared with 17% of men without a history of vasectomy, were misdiagnosed with prostate cancer by local pathologists. CONCLUSION Vasectomy is not associated with an increased risk of prostate cancer in developing countries where the rate of the disease is low.