PURPOSE: It is known that the loss of diurnal rhythm of testosterone by age is related to late-onset hypogonadism (LOH). Currently testosterone replacement therapy (TRT) has been recommended only in men with hypogonadism. We evaluated the effectiveness and safety of TRT for men with LOH symptoms and the loss of diurnal rhythm of total testosterone but normal values of total testosterone. MATERIALS AND METHODS: We enrolled 62 patients in whom the difference in testosterone between morning and evening was lower than 108 ng/dl, whose morning values were higher than 300 ng/dl, and who were diagnosed with LOH using the Androgen Deficiency in Aging Male (ADAM) questionnaire. Among the 62 patients enrolled, 44 completed the daily application of 1% testosterone gel or the intramuscular injection of long-acting testosterone undecanoate for the full 20-week period. We compared the data at baseline, and the 8th and 20th week 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 testosterone, prostate specific antigen (PSA), complete blood cell count (CBC), and lipid profile. RESULTS: The mean age was 54.9+/-7.2 years. Subjects main symptoms were sexual dysfunction and decrease of ejaculate volume. AMS scales before and after TRT were 41.3+/-18.5 and 35.8+/-19.7 (p<0.05). IIEF total scores before and after TRT were 29.7+/-13.7 and 38.9+/-17.4 (p<0.001). However, 18 patients (40.9%) were not satisfied with TRT and only 11% were fully satisfied. Total testosterone and estradiol were higher after TRT but the other values had not changed. The most common adverse event (27.3%) was erythrocystosis (18.2%). CONCLUSIONS: TRT could induce total testosterone to reach the mid-normal level and was relatively effective for aging male symptoms and sexual function. It is essential for physician to inform patients about potential adverse events and the low satisfaction rate associated with TRT even though TRT has generally been effective.