Sex steroid hormones are known to be important regulators of the lipid and glucose metabolism. Lower levels of testosterone (T) or sex hormone-binding globulin (SHBG) have been reported in men with type 2 diabetes. On the other hand, the relationship between relative hypogonadism and metabolic syndrome has not yet to be thoroughly studied. Ninety-eight Japanese adult (age 20-64) male patients with impaired glucose tolerance (IGT) or type 2 diabetes mellitus were divided into either an metabolic syndrome group (n = 42) or a non- metabolic syndrome (n = 56) group according to the definition of metabolic syndrome from WHO, or into three tertiles according to their sex hormone index level. The metabolic syndrome group had a significantly lower T/estradiol (E(2)) and SHBG level (p < 0.01). The age and subcutaneous fat surface area (SFA) were significantly different within the tertiles in SHBG and T/E(2). Logistic regression analyses were performed to investigate the association between the sex steroid hormone index level and the incidence of metabolic syndrome. Regarding the highest tertiles as a criterion, lower SHBG, T/E(2) or free T/E(2) had a higher odds ratio of prevalence of metabolic syndrome even after adjusting for age and SFA. Relative hypogonadism was strongly associated with the prevalence of metabolic syndrome in Japanese adult men who were newly diagnosed to have IGT or type 2 diabetes.