The Memory and Medical Care Study (MMCS) is a community-based, longitudinal study of elders at risk for dementia. This paper describes the study methods for identifying subjects with dementia or mild cognitive impairment (MCI) and the validation of these methods. The MMCS cohort was established by identifying subjects at risk for dementia in three previous studies of randomly ascertained samples. Neuropsychologic test score criteria were established to identify MMCS subjects with dementia or MCI. These criteria were validated using a fourth community-based sample of at-risk elders in which dementia was identified by a clinical adjudication panel. Of the 498 MMCS subjects, 70% had dementia and 27% had MCI by the MMCS criteria. In the validation sample, the MMCS dementia classification method was in agreement with the clinical adjudication panel for 81% of cases (kappa = 0.62, 95% confidence interval = 0.45–0.78). The methods used in the MMCS are efficient and reasonably valid for establishing a cohort of subjects to investigate how dementia is assessed, diagnosed, and treated in the community.