Sex hormone binding globulin (SHBG) concentrations were measured by immunoradiometric assay in serum samples from 1,221 healthy female volunteers aged 34-79 who had never used oral contraceptives or hormone replacement therapy, had no history of thyroid disease or cancer, and had not used any drugs known to influence SHBG in the 14 days preceding blood sampling. There were 616 premenopausal and 605 naturally postmenopausal women. In both premenopausal and postmenopausal women, SHBG decreased with increasing weight (Quetelt's Index) and was lower in single nulliparous women than in married nulliparous women or parous women. In premenopausal women, SHBG was higher in women with late menarche, was higher in smokers than in non-smokers, and was higher in blood samples taken during the first 12 days of the luteal phase than during the rest of the menstrual cycle. In postmenopausal women, SHBG increased with years since the menopause. The possible biological importance of these findings is discussed with particular reference to risk factors for breast cancer.