IMPORTANCE: Postmenopausal hormone therapy with conjugated equine estrogens (CEEs) may adversely affect older women's cognitive function. It is not known whether this extends to younger women. OBJECTIVE: To test whether prescribing CEE-based hormone therapy to postmenopausal women aged 50 to 55 years has longer-term effects on cognitive function. DESIGN: Trained, masked staff assessed participants with an annual telephone-administered cognitive battery that included measures of global and domain-specific cognitive functions. Cognitive testing was conducted an average of 7.2 years after the trials ended, when women had a mean age of 67.2 years, and repeated 1 year later. Enrollment occurred from 1996 through 1999. SETTING: Forty academic research centers. PARTICIPANTS: The study population comprised 1326 postmenopausal women, who had begun treatment in 2 randomized placebo-controlled clinical trials of hormone therapy when aged 50 to 55 years. INTERVENTION: The clinical trials in which the women had participated had compared 0.625 mg CEE with or without 2.5 mg medroxyprogesterone acetate over a mean of 7.0 years. MAIN OUTCOMES AND MEASURES: The primary outcome was global cognitive function. Secondary outcomes were verbal memory, attention, executive function, verbal fluency, and working memory. RESULTS: Global cognitive function scores from women who had been assigned to CEE-based therapies were similar to those from women assigned to placebo: mean (95% CI) intervention effect of 0.02 (-0.08 to 0.12) standard deviation units (P = .66). Similarly, no overall differences were found for any individual cognitive domain (all P > .15). Prespecified subgroup analyses found some evidence that CEE-based therapies may have adversely affected verbal fluency among women who had prior hysterectomy or prior use of hormone therapy: mean treatment effects of -0.17 (-0.33 to -0.02) and -0.25 (-0.42 to -0.08), respectively; however, this may be a chance finding. CONCLUSIONS AND RELEVANCE: CEE-based therapies produced no overall sustained benefit or risk to cognitive function when administered to postmenopausal women aged 50 to 55 years. We are not able to address whether initiating hormone therapy during menopause and maintaining therapy until any symptoms are passed affects cognitive function, either in the short or longer term. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT01124773.