The distribution and morphology of senile plaques (SPs) in the cerebral cortices and subcortical nuclei of six cases of familial Alzheimer's disease (AD) were examined using the Methenamine-Bodian method and compared with those of sporadic AD cases. SPs were grouped into three types according to their morphology. SP types were generally constant at each anatomical site in all of the cases. The SPs of familial cases, however, had a greater tendency to fuse together than those of sporadic cases, especially in the cingulate cortex, presubiculum and striatum. This tendency was more evident in cases with severe amyloid angiopathy. Here it appeared that a SP type corresponding to "diffuse plaques", at least in part, might be formed by transformation from another type. In the globus pallidus, all the familial cases had many compact-like plaques which appeared to be derived from "drusige Entartung" of the capillaries. Furthermore, the regional proportion of two types of SPs occurring in this nucleus varied along its anteroposterior axis. These findings may be the histological hallmarks of atypical AD rather than familial AD.