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 involved. Isolated SPs were seen in only 3 brains (6%); SPs and NFTs coexisted in 13 (25.5%). The distribution patterns of NFTs and SPs differed. NFTs were seen in discrete, mostly single neurons of the structure, already noted, whereas SPs occurred in a more generalized distribution over the base and convexity of the cerebrum. The plaques were usually small (30 mu 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.