To determine whether free (or active) testosterone concentrations are reduced in men receiving glucocorticosteroids for chronic inflammatory diseases, 17 men (mean age 55.5 years) receiving a mean daily dose of 16.3 mg prednisolone, and 13 control patients (mean age 52.2 years) receiving no prednisolone, were studied. Serum testosterone and the testosterone/sex hormone binding globulin (SHBG) ratio were measured. The testosterone/SHBG ratio (a measure of free (active) testosterone) was significantly reduced in patients treated with prednisolone (p = 0.026), thus showing that glucocorticosteroids reduce free testosterone in male patients. This may be an important cause of glucocorticosteroid-induced osteoporosis, and suggests an additional approach to bone prophylaxis and treatment.