PURPOSE: Currently, no studies have examined the diurnal variation of sex hormones in Korean men. We realized that accurate models of the diurnal variation of the sex hormones in Korean males are needed to evaluate and manage patients in urology. Therefore diurnal variation of testosterone and related sex hormones were investigated in young Korean men. MATERIALS AND METHODS: Healthy Korean men from 20 to 25 years old who had evident secondary growth were enrolled. We drew blood from each subject 12 times at 2-hour intervals for 24 hours. The concentrations of total testosterone, estradiol, luteinizing hormone, sex hormone binding globulin and albumin were measured. The free and bioavailable testosterone were calculated by Vermeulen's equation. To analyze the rhythmicity of diurnal variation in sex hormones, 24-hr cosine regression analysis was used for the statistical analysis. RESULTS: Total testosterone, free testosterone, estradiol, and testosterone estradiol ratio demonstrated a significant diurnal cosinor rhythm. The diurnal variation of bioavailable testosterone did not demonstrate a statistically significant cosinor rhythm. CONCLUSIONS: The diurnal variations of serum total and free testosterone concentration in Korean young men were similar to those found in studies of Caucasian men. Our study provides basic data for future studies of Korean male sex hormones. Furthermore, additional studies targeted toward various male age groups are needed to make the standard models of sex hormones in Korean men.