We report a longitudinal study of 16 subjects originally enrolled in the Washington University (St Louis, Mo) Memory and Aging Project with Clinical Dementia Rating (CDR) of "questionable" dementia (CDR 0.5). A 0.5 rating was intended to characterize subjects in whom mild cognitive impairment due to senile dementia of the Alzheimer type was suspected but was insufficient in degree to warrant a diagnosis of definite dementia. Over an 84-month follow-up period, 11 of the 16 subjects either had Alzheimer's disease verified post mortem or had clinically progressed to a more advanced CDR stage in which the dementia was clearly evident. These results suggest that the CDR 0.5 stage likely represents the incipient clinical manifestation of Alzheimer's disease and that the majority of subjects with CDR 0.5 have "very mild senile dementia of the Alzheimer type." Performance on several standard clinical scales was significantly different when comparing a larger sample of controls (n = 83), subjects with CDR 0.5 (n = 41), and subjects with mild senile dementia of the Alzheimer type (score of 1; n = 68).