Background: Dutasteride is an inhibitor of both types I and II 5 alpha-reductase and was approved in Korea in April 2004. This post-marketing surveillance was to assess the safety of dutasteride in Korean patients with benign prostate hyperplasia in real life and to elucidate the risk factors related adverse events. Methods: From December 2004 to January 2010, 3,977 patients were enrolled by 184 urologists. According to post-marketing surveillance regulation, patients were enrolled consecutively. Patients administered dutasteride at least once were included in safety assessment. The incidences of any adverse events and serious adverse events were evaluated. Multiple logistic regression method was used to identify risk factors related to adverse events. Results: The safety assessment included 3,870 patients with the mean age of 67.3 years. The incidence of adverse events was 3.8%. The most frequent adverse event was impotence (75 cases, 1.9%), libido decrease (49 cases, 1.3%), ejaculation disorder (30 cases, 0.8%), and gynecomastia (5 cases, 0.1%). The incidence of unexpected adverse events was 0.5% and cerebral infarction, lung cancer, pulmonary embolism, and diarrhea were reported as serious adverse events. Conclusion: In this survey, impotence was the most frequently reported adverse events. Dutasteride was well tolerated in Korean patients with benign prostate hyperplasia. These results updated the safety information and would provide important additional information for prescribers.