In a previous study of hippocampal neurons in aging and AD [Lancet 344 (1994) 769], we demonstrated that the loss of neurons in the CA1 region was disease-specific and not related to aging. In the present study, we examined for loss of hippocampal neurons in preclinical AD, a period during which there are abundant amyloid deposits in the brain but no evidence of cognitive decline. We examined the postmortem brains of 33 subjects from the Baltimore Longitudinal Study of Aging and the Johns Hopkins Alzheimer's Disease Research Center. Using unbiased stereology, we estimated the total number of neurons in the granule cell layer, hilus, CA3-2, CA1, and subiculum of AD (n = 14) preclinical AD (n = 8), and age-matched control subjects (n = 11). The results from the present study confirm our previous finding of significant neuronal losses in the CA1 (48%), hilus (14%), and subiculum (24%) in AD [Lancet 344 (1994) 769]. However, we did not observe a significant loss of neurons in CA1 or any of the other subdivisions of the hippocampus in preclinical AD.