To assess whether testosterone (T) supplementation in men considered to have symptomatic late-onset hypogonadism (SLOH) can be evaluated clinically and biochemically. To assess the relevance of the clinical and biochemical diagnosis of hypogonadism we investigated patients referred for the diagnosis and treatment of SLOH. Patients were assessed clinically and completed a screening questionnaire. The pituitary-adrenal-gonadal axis was comprehensively assessed biochemically. Those with a clinical diagnosis of hypogonadism and serum levels of T supporting such a diagnosis received exogenous T for >/= 3 months and were assessed for any clinical and biochemical response. Of an initial group of 45 men (mean age 59.2 years) 38 completed the study. Most men presented with symptoms of sexual dysfunction, lack of energy and/or depression. There were differences before and after treatment only in bioavailable T (BT), with none in the levels of total T (TT). There was a strong correlation before and after treatment in the levels of luteinizing hormone and follicle-stimulating hormone, and a weak negative correlation between gonadotrophins and BT. Neither TT nor BT had predictive value for the treatment response. There was a trend to a correlation between BT levels and treatment success. Changes in serum prostate specific antigen were insignificant during the limited period. The lack of accurate methods for diagnosing SLOH suggests that a therapeutic trial of T supplementation is warranted in men in whom there are no contraindications. The 3-month period largely circumvents the placebo effect and has minimal risks for serious adverse effects (mostly in relation to prostate safety). This controversial position needs further evaluation with a larger cohort and other biochemical measurements.