The Modified Mini Mental State Exam (3MS) is widely used for screening global cognitive functioning, however little is known about its performance in clinical trials. We report the distribution of 3MS scores among women enrolled in the Women's Health Initiative Memory Study (WHIMS) and describe differences in these scores associated with age, education, and ethnicity. The 3MS exams were administered to 7,480 women aged 65-80 who had volunteered for and were eligible for a clinical trial on postmenopausal hormone therapy. General linear models were used to describe demographic differences among scores. Factor analysis was used to characterize the correlational structure of exam subscales.The distribution of 3MS scores at baseline was compressed in WHIMS compared to population-based data. Mean 3MS scores (overall 95.1) tended to decrease with age and increase with education, however these associations varied among ethnic groups (p< 0.0001) even after adjustment for health, physical disability and occupation attainment. Four factors accounted for 37% of the total variance. Each varied with education and ethnicity; the two most prominent factors also varied with age. Despite relatively narrow distributions in WHIMS, baseline 3MS scores retained associations with age and education. These associations varied among ethnic groups, so that care must be taken in comparing data across populations.