The aim of this study was to determine the prevalence and investigate the aetiology of hypogonadism in men on methadone or buprenorphine maintenance treatment (MMT, BMT). 103 men (mean age 37.6 +/- 7.9) on MMT (n = 84) or BMT (n = 19) were evaluated using hormone assays, body mass index (BMI), serological, biochemical, demographic and substance use measures. Overall 54% of men (methadone 65%; buprenorphine 28%) had total testosterone (TT) <12.0 nm; 34% (methadone 39%; buprenorphine 11%) had TT <8.0 nm. Both methadone- and buprenorphine-treated men had lower free testosterone, luteinising hormone and estradiol than age-matched reference groups. Methadone-treated men had lower TT than buprenorphine-treated men and reference groups. Prolactin did not differ between methadone, buprenorphine groups, and reference groups. Primary testicular failure was an uncommon cause of hypogonadism. Yearly percentage fall in TT by age across the patient group was 2.3%, more than twice that expected normally. There were no associations between TT and opioid dose, cannabis, alcohol and tobacco consumption, or chronic hepatitis C viraemia. On multiple regression higher TT was associated with higher alanine aminotransferase and lower TT with higher BMI. Men on MMT have high prevalence of hypogonadotrophic hypogonadism. The extent of hormonal changes associated with buprenorphine needs to be explored further in larger studies. Men receiving long term opioid replacement treatment, especially methadone treatment, should be screened for hypogonadism. Wide interindividual differences in methadone metabolism and tolerance may in a cross-sectional study obscure a methadone dose relationship to testosterone in individuals. Future studies of hypogonadism in opioid-treated men should examine the potential benefits of dose reduction, choice of opioid medication, weight loss, and androgen replacement.