We developed a highly sensitive assay for estrone and 17 beta-estradiol in serum. Estrone and 17 beta-estradiol, obtained by solid-phase extraction using a Sep pak tC18 cartridge, were purified by high-performance liquid chromatography (HPLC). Quantitation of estrone and 17 beta-estradiol were carried out by radioimmunoassay. Not insignificantly, this automatic system of extraction and HPLC succeeded in analyzing 80 samples a week. Intra-assay coefficients of variation (CV) for estrone and 17 beta-estradiol ranged from 19.5 to 28.7%, and from 8.5 to 13.7%, respectively. The minimum detectable dose for estrone and 17 beta-estradiol were 1.04 pg/ml and 0.64 pg/ml, respectively. The serum levels of 17 beta-estradiol using our method strongly correlated with those by Gas chromatography mass spectrometry (GC-MS). The serum levels of estrone and 17 beta-estradiol in 154 peri- and postmenopausal women were estimated to be between 15 and 27 pg/ml and between 3.5 and 24.0 pg/ml, respectively, while the serum level of 17 beta-estradiol in postmenopausal women, in particular, was estimated to be from 3.5 to 6.3 pg/ml. For postmenopausal women who suffered from vasomotor symptoms, the mean levels of estrone and 17 beta-estradiol at 12 to 18 hours after treatment with daily 0.625 mg conjugated equine estrogen (CEE) and 2.5 mg medroxyprogesterone acetate (MPA) were 135.0 and 21.3 pg/ml at 12 months, respectively. On the other hand, levels of estrone and 17 beta-estradiol at 12 to 18 hours after treatment with CEE and MPA every other day, were 73.4 and 15.3 pg/ml, respectively. These highly sensitive assays for estrone and 17 beta-estradiol are useful in measuring low levels of estrogen in postmenopausal women, and monitoring estrogen levels in women receiving CEE as hormone replacement therapy.