Most epidemiologic studies of plasma lipoproteins and gonadal steroids have measured total hormone concentrations only. Gonadal steroids are transported in plasma in unbound, albumin-bound and sex hormone-binding globulin (SHBG)-bound forms, only the first two of which are biologically active. In the present study the associations of plasma lipoproteins with the different fractions of testosterone, oestradiol and 5 alpha-dihydrotestosterone (5 alpha DHT) were explored by multiple regression in 70 men aged 52-67 years (mean, 59 years). The principal finding was that the distribution of cholesterol between the two major subclasses of high density lipoprotein (HDL) was correlated with the concentrations of unbound and albumin-bound oestradiol, but not with SHBG-bound oestradiol or any fraction of testosterone or 5 alpha DHT. These associations were independent of plasma triglyceride. Apoproteins A-I and A-II were not correlated with any hormone fraction. Thus, in middle-aged/elderly men HDL subclasses appear to be influenced by circulating estrogenic activity, but not by androgenic activity, through a mechanism that is unrelated to triglyceride transport or HDL apoprotein metabolism.