OBJECTIVE: Measurement of serum testosterone is an integral part of the assessment of men presenting to endocrine clinics. Little is known about the variation of total bound or bioavailable testosterone by ethnic group. The principal determinant of testosterone bioavailability is SHBG, which itself is a marker for insulin sensitivity. Our aim was to examine variations in testosterone and SHBG levels across three ethnic groups in relation to ethnic differences in insulin sensitivity. DESIGN: Men of three ethnic groups living in Manchester, UK, were sampled randomly from population registers being of white European (n = 55), Pakistani (n = 50) and African-Caribbean (AfC) origin (n = 75). Circulating serum testosterone and SHBG concentrations were measured and free testosterone calculated. Insulin sensitivity (HOMA-S) and insulin secretory capacity (HOMA-B) were determined from fasting plasma intact insulin and glucose values. RESULTS: Testosterone levels were lower in Pakistani men (mean 14.6 nmol/l, 95% confidence interval 12.6-16.6 nmol/l) than in Europeans (18.7, 16.8-20.6 nmol/l) or AfCs (18.0, 16.4-19.6 nmol/l) (F = 4.8, P = 0.009). Despite SHBG levels also being lower in Pakistani men (22.9, 19.4-26.5 nmol/l) compared with Europeans (28.7, 25.7-31.8 nmol/l) and AfCs (26.9, 23.9-30.0 nmol/l) (F = 3.0, P