OBJECTIVES: To examine the effects of vitamin D and calcium on cognitive outcomes in elderly women. DESIGN: Post hoc analysis of a randomized double-blind placebo-controlled trial. SETTING: Forty Women's Health Initiative (WHI) clinical centers across the United States. PARTICIPANTS: Four thousand one hundred forty-three women aged 65 and older without probable dementia at baseline who participated in the WHI Calcium and Vitamin D Trial and the WHI Memory Study. INTERVENTION: Two thousand thirty-four women were randomized to receive 1,000 mg of calcium carbonate combined with 400 IU of vitamin D(3) (treatment) and 2,109 to placebo. MEASUREMENTS: Primary: classifications of probable dementia or mild cognitive impairment (MCI) based on a four-phase protocol that included central adjudication. Secondary: global cognitive function and individual cognitive subtests. RESULTS: Mean age of participants was 71. During a mean follow-up of 7.8 years, 39 participants in the treatment group and 37 in the placebo group developed incident dementia (hazard ratio (HR) = 1.11, 95% confidence interval (CI) = 0.71-1.74, P = .64). Likewise, 98 treatment participants and 108 placebo participants developed incident MCI (HR = 0.95, 95% CI = 0.72-1.25, P = .72). There were no significant differences in incident dementia or MCI or in global or domain-specific cognitive function between groups. CONCLUSION: There was no association between treatment assignment and incident cognitive impairment. Further studies are needed to investigate the effects of vitamin D and calcium separately, on men, in other age and ethnic groups, and with other doses.