This is a multicenter double-blind, controlled clinical trial in which 60 subjects with a primary dementia (probable Alzheimer's disease [AD] or probable dementia with Lewy bodies [DLB]) and coexisting parkinsonism, or Parkinson's disease with dementia [PDD] will be randomized to 1 of 2 treatment groups: (1) quetiapine (QUET); an atypical antipsychotic with a favorable extrapyramidal side effect profile), or (2) placebo. Each subject participates in the trial for 10 weeks and systematic ratings of behavior, motor function, cognition, adverse events and other outcomes occur at baseline and after 6 and 10 weeks of assigned treatment.