Purpose: The risk of prostate cancer after vasectomy was studied using population based register data from Denmark. Materials and Methods: Men hospitalized for vasectomy in Denmark from 1977 to 1989 were followed for death, emigration and incident cancer by administrative registers. The observed number of patients with cancer in the cohort was divided by the number expected based on the cancer incidence rate for all Danish men, providing a standardized incidence ratio. Results: A total of 46 cohort patients were diagnosed with prostate cancer, while 46.93 were expected (standardized incidence ratio 0.98, 95% confidence interval 0.7 to 1.3). Time since vasectomy or age at vasectomy showed no trend. Conclusions: This population based Danish cohort study indicated no increase in the risk of prostate cancer in men hospitalized for vasectomy. The study was free of reporting bias since all data were collected from administrative registers. The cohort was not comprehensive but it most likely represented an unbiased sample of Danish men who underwent vasectomy. The study strongly adds to the evidence that there is no excess prostate cancer risk after vasectomy is done for sterilization.