OBJECTIVE: To assess the effect of hormone therapy (HT) on coronary heart disease (CHD) events in younger and older postmenopausal women. DESIGN: A comprehensive database search identified randomized-controlled trials of HT of at least 6 months' duration that reported CHD events, defined as myocardial infarction or cardiac death. MEASUREMENTS: The pooled odds ratios (ORs) for CHD events were reported separately for younger and older women, defined as participants with mean time from menopause of less than or greater than 10 years, or mean age less than or greater than 60 years. MAIN RESULTS: Pooled data from 23 trials, with 39,049 participants followed for 191,340 patient-years, showed that HT significantly reduced CHD events in younger women (OR 0.68 [confidence interval (C I), 0.48 to 0.96]), but not in older women (OR 1.03 [CI, 0.91 to 1.16]). Hormone therapy reduced events in younger women compared with older women (OR 0.66 [CI, 0.46 to 0.95]). In older women, HT increased events in the first year (OR 1.47 [CI, 1.12 to 1.92]), then reduced events after 2 years (OR 0.79 [CI, 0.67 to 0.93]). CONCLUSIONS: Hormone therapy reduces the risk of CHD events in younger postmenopausal women. In older women, HT increases, then decreases risk over time.