Purpose: We evaluated the efficacy and safety of transdermal 1% testosterone gel in Korean men with late-onset hypogonadism from Aug. 2004 to Dec. 2005 in a prospective, fixed-dose, placebo-uncontrolled, open label, multicenter study. Materials and Methods: From the 5 urological centers in Korea, a total of 87 male patients older than 40 years who were diagnosed with late-onset hypogonadism with serum testosterone lower than 350 ng/dl or serum free testosterone lower than 73.5 pg/ml were enrolled. Among the 87 enrolled 68 patients completed the daily application of 1% testosterone gel according to the standard consumer guidelines for the full 12-week period. We compared the data at baseline and at 4, 8, and 12 weeks follow-up using the Aging Males'' Symptoms (AMS) scale, the International Index of Erectile Function (IIEF)-15, the International Prostate Symptoms Score (IPSS), and the serum levels of total and free testosterone and prostate specific antigen (PSA).