Hyperandrogenicity in women is closely associated with insulin resistance and a risk factor for cardiovascular disease and noninsulin-dependent diabetes mellitus (NIDDM). Therefore, 25 postmenopausal women with NIDDM and sex hormone-binding globulin values less than 60 nmol/L, as an indicator of a moderate hyperandrogenicity, were treated with 2 mg 17-beta-estradiol orally for 3 months in a double-blind, cross-over, placebo-controlled trial. During the last 16 days of active treatment, 1 mg norethisterone acetate was added for 10 days for endometrial protection. Blood glucose, glycosylated hemoglobin, insulin, c-peptide, lipoprotein profile, sex steroid hormones, GH, and insulin-like growth factor I (IGF-I) were measured, and insulin sensitivity was determined by the euglycemic hyperinsulinemic clamp method. All metabolic measurements were taken at baseline and after 68 days of active or placebo treatment. Estradiol treatment, compared with the placebo period, was followed by a marked increase of sex hormone-binding globulin and a decrease of free testosterone. Blood glucose, glycosylated hemoglobin, c-peptide, total cholesterol, low-density lipoprotein cholesterol, and IGF-I decreased significantly (P < 0.01-P < 0.001), whereas high-density lipoprotein cholesterol rose (P < 0.001). In conclusion, estrogen replacement therapy in postmenopausal women with NIDDM ameliorated hyperandrogenicity, and this was accompanied by marked improvements in glucose homeostasis and lipoprotein profile.