OBJECTIVES: To determine the baseline serum concentrations of estradiol and estrone in postmenopausal Korean women and the serum concentrations of estradiol and estrone after 4 and 16 weeks of treatment using 1 mg of estradiol and 2 mg of drospirenone. METHODS: This was a multicenter study. Thirty-six subjects were screened. Serum estradiol, estrone and drospirenone levels were determined by gas chromatography-mass spectrometry and radioimmunoassay. RESULTS: The mean estradiol concentration was 8.37 +/- 12.1 pg/mL at baseline and increased to 53.7 +/- 52.1 and 41.4 +/- 26.1 pg/mL after 4 and 16 weeks of treatment, respectively. The mean estrone concentrations were 28.7 +/- 26.8, 266.1 +/- 182.9, and 256.1 +/- 179.1 pg/mL at baseline, and after 4 and 16 weeks of treatment, respectively. When women were stratified according to the basal estradiol level, the level after 4 weeks of treatment was significantly higher in the women with a detectable level (> or = 5 pg/mL) than in women with an undetectable level (< 5 pg/mL; 65.2 +/- 21.5 vs. 37.4 +/- 25.8 pg/mL, P = 0.008). After 16 weeks of treatment, the estradiol level was still higher in the detectable group (51.6 +/- 28.6 vs. 38.7 +/- 21.7 pg/mL, P = 0.09). CONCLUSION: This study showed that 1 mg of estradiol and 2 mg of drospirenone is an appropriate regimen to achieve the desired serum estradiol level. The difference in serum hormonal levels after 4 weeks of treatment could be caused by different basal levels.