The distribution of amyloid beta peptide (A beta) was quantified in the corpus striatum and pallidum of 10 patients with Alzheimer's disease (AD) and three patients with both Alzheimer's disease and Parkinson's disease (AD-PD). A beta occurred almost exclusively in plaques that did not have neurites or amyloid cores. Caudate, accumbens nuclei and rostral putamen contained more of the diffuse plaques than did caudal putamen. No diffuse plaques were found in the neighbouring globus pallidus. This distribution of A beta deposition may reflect the distribution of diseased synaptic cortical afferents rather than a putative vascular source of A beta.