Senile plaques (SP) and neurofibrillary tangles (NFT) were found in 38 (74.5%) of 51 unselected brains of nondemented patients who died between the ages of 55 and 64 years. A high proportion (22, or 43%) had only NFTs. These were consistently present in the entorhinal cortex and/or the hippocampus; the olfactory bulb, the amygdala, and the nucleus basalis of Meynert were also occasionally invloved. Isolated SPs were seen in only 3 brains (6%); SPs and NFTs coexisted in 13 (25.5%). The distribution patterns of NFTs and SPs occurred in a more generalized distribution over the base and convexity of the cerebrum. The plaques were usually small (30 μ in diameter) and consisted of delicate fibrillary material. Other types of SP were also seen. The incidence of various types of plaques in nondemented patients is considered to indicate a morphological evolution of these structures.