Estrogen-replacement therapy is important for the prevention of postmenopausal osteoporosis. However, oral synthetic and conjugated estrogens increase biliary cholesterol saturation index and risk of gallstone disease. To examine whether transdermal estrogen administration could avoid these adverse effects, 17 postmenopausal women were treated with transdermal estradiol (Estraderm TTS; Ciba-Geigy, Arnhem, The Netherlands), 100 micrograms/day for 4 weeks, and after 1 month without therapy, with oral estradiol (Progynova; Schering, Weesp, The Netherlands), 2 mg/day for 4 weeks. The increase in the serum estradiol level was much higher during transdermal than oral estradiol administration. On the contrary, the increase in the serum estrone level was much more pronounced during oral treatment. Both modes of treatment led to a similar reduction of urinary calcium excretion. A highly significant decrease in serum phosphate levels was found during transdermal therapy. Biliary cholesterol saturation index did not change during transdermal therapy (mean +/- SEM, 1.25 +/- 0.06 before and 1.22 +/- 0.07 at the end of transdermal therapy; P = NS). A slight increase in cholesterol saturation index that did not reach statistical significance was found during oral therapy (1.28 +/- 0.09 before and 1.36 +/- 0.09 during oral treatment). However, the subgroup of women with strong increases in serum estrone levels during oral estradiol therapy (greater than 0.5 pmol/mL; n = 8) generally had increased biliary cholesterol saturation index, a decrease in relative percentage chenodeoxycholic acid in bile, and increased serum sex hormone-binding globulin levels during oral treatment. Cholesterol monohydrate crystals were never found in duodenal biles during either treatment. This study indicates that transdermal estradiol does not induce lithogenic bile. On the contrary, oral estradiol leads to lithogenic bile in a subgroup of women with strong increases in serum estrone levels during oral treatment.