Background: The population of oldest old is increasing, but the prevalence of cognitive impairment is not well characterized in this group. Objectives: To determine the prevalence of mild cognitive impairment (MCI), dementia, and their subtypes in oldest old women and to examine whether some groups of oldest old women were more likely to have cognitive impairment. Design: Prospective cohort study. Setting: Women Cognitive Impairment Study of Exceptional Aging. Participants: A total of 1299 oldest old (≥85 years) women. Main Outcome Measures: All the women completed a neuropsychological test battery. Those who screened positive for possible cognitive impairment (n=634) were further assessed for a diagnosis of dementia, MCI, or normal cognition. The remaining women (n=665) were considered cognitively normal. Dementia and MCI subtypes were determined using standard criteria. Results: The women had a meanage of 88.2 years, and 27.0% were 90 years or older; 231 women (17.8%) were diagnosed as having dementia and 301 (23.2%) as having MCI, for a combined cognitive impairment prevalence of 41.0%. Clinical features consistent with Alzheimer disease and mixed dementia were most common, each accounting for 40% of dementia cases. Amnestic multiple domain and nonamnestic single domain were the most common MCI types, accounting for 33.9% and 28.9% of cases, respectively. Cognitive impairment was more frequent in women 90 years or older compared with those 85 to 89 years (dementia, 28.2% vs 13.9%, P<.001; MCI, 24.5% vs 22.7%, P=.02) and was more common in women with less education, a history of stroke, and prevalent depression. Conclusions: In this large sample of oldest old women, 41.0% had clinically adjudicated cognitive impairment. Subtypes of dementia and MCI were similar to those in younger populations. Women in the fastest growing demographic, the oldest old, should be screened for cognitive disorders, especially high-risk groups.