The distribution of oestradiol in plasma from postmenopausal women with or without breast cancer has been measured and related to the metabolic clearance rates of oestradiol (MCR-E2). All measurements were carried out preoperatively. No difference in the free fraction of oestradiol was found between women with breast cancer (1.8 +/- 0.4%, mean +/- SD) and normal women (1.6 +/- 0.3%). Approximately 40% of oestradiol in plasma was bound to albumin with 60% bound to sex hormone binding globulin (SHBG), but values were similar for women with or without breast cancer. Concentrations of total unconjugated oestradiol and free oestradiol (2.4 +/- 0.5 ng/100 ml and 44 +/- 12 pg/100 ml respectively, mean +/- SD, n = 9) were significantly higher in plasma from postmenopausal women with breast cancer compared with levels in plasma from normal postmenopausal women (1.8 +/- 0.5 ng/100 ml and 27 +/- 10 pg/100 ml, n = 8). A significant correlation was found between the free oestradiol fraction and MCR-E2 (r = 0.53, p less than 0.05). Correlations were also found between the fractions of oestradiol bound to albumin and MCR-E2 (r = 0.45). From correlations between the binding capacity of SHBG and MCR-E2 it appears that, for a given level of SHBG, MCR-E2 may be higher in women with breast cancer.