Post-mortem pathological and biochemical studies are reported on six patients with progressive dementia. The characteristic pathological finding was neurofilament-containing cytoplasmic inclusions in cortical and subcortical neurons. The clinical and pathological findings were consistent with so-called diffuse Lewy body disease. The patients had variable changes of the Alzheimer type, with five of six patients displaying "plaques only" Alzheimer's changes. Biochemical studies showed profound decreases in neocortical choline acetyltransferase (ChAT) activities that correlated with marked neuronal loss in the basal nucleus of Meynert. ChAT activities were normal in the hippocampus in three patients who also had no significant Alzheimer type hippocampal changes. All patients had decreased cortical somatostatin-like immunoreactivity. Our observations suggest that dementia in diffuse Lewy body disease bears biochemical similarities to Alzheimer's disease, in that biochemical markers for both intrinsic cortical neurons and ascending cholinergic neurons are affected.