Although neuritic and diffuse plaques generally coexist in Alzheimer disease (AD) neocortex, the predominance of diffuse plaques in the striatum prompted us to explore the potential influence of striatal features such as the striatal mosaic on beta-amyloid (A beta) deposition. Using double immunohistochemistry with an antibody to A beta in combination with antibodies to met-enkephalin or somatostatin, we investigated the relationship between diffuse plaques and neuropeptide distribution in the dorsal striatum. This relationship was examined in "pure" AD cases as well as in AD cases with coexisting Parkinson disease (PD) pathology, i.e. nigral degeneration and Lewy bodies at any site (AD + PD). Despite the presence of numerous A beta-positive diffuse plaques in both groups, the mosaic pattern, as exemplified by met-enkephalin-immunoreactive patches, was preserved. No obvious association was observed between the plaques and met-enkephalin-positive patches or somatostatin-immunoreactive neurons. Tyrosine hydroxylase immunoreactivity in the matrix was, however, diminished in AD + PD, most likely reflecting the nigral degeneration in these cases. Overall, these observations suggest that neostriatal A beta deposition in AD is not influenced by environmental factors associated with the striatal mosaic.