OBJECTIVE To see whether the short-term changes in serum lipid and lipoprotein concentrations induced by postmenopausal estrogen-progestin therapy are maintained in the long term. METHODS Sixty-one healthy postmenopausal women were randomized to either oral therapy (continuous conjugated equine estrogens at 0.625 mg/day with sequential dl-norgestrel at 0.15 mg/day for 12 days each cycle) or transdermal therapy (patches delivering continuous 17 beta-estradiol [E2] at 0.05 mg/day with sequential norethindrone acetate at 0.25 mg/day for 14 days each cycle). Twenty-nine healthy postmenopausal women who did not request therapy served as a reference group. Fasting serum lipid and lipoprotein concentrations were monitored for 3 years. RESULTS Studied in the estrogen-progestin phase, oral and transdermal therapies reduced serum total cholesterol concentrations by 12.1% (P < .001) and 8.4% (P < .001), respectively, and those of low-density lipoprotein (LDL) by 14.2% (P < .001) and 6.6% (P < .01), respectively. These changes, apparent at 3 months, were maintained over 3 years. Serum triglyceride concentrations fell by 2.5% (P < .05) and 16.4% (P < .01), respectively. These decreases were evident after 6 months in both groups but were maintained over 3 years only in the transdermal group. High-density lipoprotein (HDL) concentrations fell in women given oral therapy (7.8%, P < .05) and transdermal therapy (10.7%, P < .001), as well as in untreated women (7.0%, P < .05). CONCLUSIONS The potentially beneficial effects of estrogen-progestin therapy on serum total and LDL cholesterol and on triglycerides were maintained over 3 years. Interpretation of the potentially detrimental effects on HDL concentrations was hindered by the changes seen in untreated women.