This paper reports neuropsychologic and pathologic data for eleven patients with a clinical diagnosis of probable Alzheimer disease (AD) according to recently proposed NINCDS criteria. In all cases, the clinical diagnoses were verified by cortical biopsy using histopathologic criteria for definite AD. Similar tissue samples from nine non-demented autopsied patients were also evaluated and none received an histopathologic diagnosis of AD. Correlations between cortical plaque counts and neuropsychologic test scores are also presented. These data have important implications for 1) the accuracy of clinical diagnosis in AD; 2) the validity of NINCDS criteria for the clinical and pathologic diagnosis of AD; and 3) the utility of cognitive test scores as indicators of disease severity.