Objectives: To determine whether smaller brain volumes in older women who had completed Women's Health Initiative (WHI)-assigned conjugated equine estrogen-based hormone therapy (HT), reported by WHI Memory Study (WHIMS)-MRI, correspond to a continuing increased rate of atrophy an average of 6.1 to 7.7 years later in WHIMS-MRI2.Methods: A total of 1,230 WHI participants were contacted: 797 (64.8%) consented, and 729 (59%) were rescanned an average of 4.7 years after the initial MRI scan. Mean annual rates of change in total brain volume, the primary outcome, and rates of change in ischemic lesion volumes, the secondary outcome, were compared between treatment groups using mixedeffect models with adjustment for trial, clinical site, age, intracranial volumes, and time between MRI measures.Results: Total brain volume decreased an average of 3.22 cm 3 /y in the active arm and 3.07 cm 3 /y in the placebo arm (p 5 0.53). Total ischemic lesion volumes increased in both arms at a rate of 0.12 cm 3 /y (p 5 0.88).Conclusions: Conjugated equine estrogen-based postmenopausal HT, previously assigned at WHI baseline, did not affect rates of decline in brain volumes or increases in brain lesion volumes during the 4.7 years between the initial and follow-up WHIMS-MRI studies. Smaller frontal lobe volumes were observed as persistent group differences among women assigned to active HT compared with placebo. Women with a history of cardiovascular disease treated with active HT, compared with placebo, had higher rates of accumulation in white matter lesion volume and total brain lesion volume. Further study may elucidate mechanisms that explain these findings. Numerous cross-sectional and observational studies on humans and animal models have examined whether postmenopausal hormone therapy (HT) affects brain structure, with inconsistent results. 1 This relationship was examined in the context of the MRI substudy of a large randomized placebo-controlled clinical trial: the Women's Health Initiative Memory Study (WHIMS)-MRI. Women aged 65 to 79 years treated for an average of 4.0 years with conjugated equine estrogen (CEE) with medroxyprogesterone acetate (MPA) or 5.6 years with CEE alone, compared with placebo, had smaller frontal lobe and hippocampal volumes on posttrial brain MRI.2 Because these findings were based on a single cross-sectional brain scan, it is unknown when the increased rate of atrophy occurred and whether it continued to increase, thereby signaling growing concerns about risks of cognitive impairment.