OBJECTIVE: To derive normal ranges of serum testosterone and luteinizing hormone (LH) concentrations in healthy men, and thus evaluate whether testosterone replacement therapy is prescribed inappropriately. SUBJECTS AND METHOD: The study comprised 266 healthy male volunteers (aged 18-75 years) who were defined as healthy by strict eligibility criteria. Subjects had a body mass index (BMI) of 18.6-32.2 kg/m2, smoked 0-10 cigarettes/day, and had an alcohol intake 0-40 units/week (one unit = 8 g ethanol). We measured serum testosterone and LH concentrations in the morning (08.00-09.00 hours) and evening (20.00-21.00 hours). RESULTS: Morning normal ranges of testosterone for men aged < or = 40 years were 10.07-38.76 nmol/L (2.90-11.18 microg/L), and for men age > or = 40 years, 7.41-24.13 (2.14-6.96); the respective evening normal ranges were 6.69-31.51 (1.93-9.09) and 6.46-21.93 (1.86-6.33). Both morning and evening serum testosterone declined significantly with increasing age and BMI. LH was significantly higher in the morning than in the evening, but did not vary between the age groups or with BMI. The calculated normal ranges of LH were 0.9-7.0 IU/L (morning) and 0.7-6.8 IU/L (evening). CONCLUSIONS: The lower limit of normal for serum testosterone was 3-4 nmol/L (0.86-1.15 microg/L) lower than that of published ranges. The results have important implications for the diagnosis of hypogonadism and use of testosterone replacement therapy.