Reproductive and menstrual characteristics, as well as high circulating estrogen concentrations, are associated with risk of hormone-related cancers in postmenopausal women. To explore possible etiologic relationships between menstrual/reproductive characteristics and risk of hormone-related cancers, we examined associations between menstrual/reproductive factors and serum concentrations of free estradiol, total estradiol, estrone, sex hormone binding globulin (SHBG), and follicle stimulating hormone (FSH). This study was conducted in 173 postmenopausal women using data from the prerandomization visit of an exercise clinical trial. Participants were sedentary, overweight/obese, and not on hormone therapy. Women > or =20 years past menopause had 23% lower total estradiol and 30% lower free estradiol concentrations than women within 4 years of menopause (P for trend = 0.04 and 0.02, respectively). Nulliparous women had 19% higher FSH concentrations than parous women (P = 0.02). Among parous women, parity was positively associated with SHBG and negatively associated with free estradiol concentrations. Women with > or =4 children had 20% lower free estradiol and 38% higher SHBG concentrations compared with women with one birth (P for trend = 0.02 and 0.01, respectively). Total number of months spent breast-feeding was modestly and inversely associated with serum FSH concentrations (P for trend = 0.07). Our results suggest that menstrual/reproductive characteristics may be associated with postmenopausal hormone concentrations; verification of these results in other studies may elucidate how these variables influence risk of hormone-related cancers.