APOE genotypes were compared in 57 cases of familial Parkinson's disease, 46 cases of sporadic Parkinson's disease, and 387 controls. The frequency of the APOE allele epsilon4 was similar in patients with Parkinson's disease and controls, but the APOE allele epsilon2, thought to be protective for dementia, was significantly more frequent in patients with sporadic Parkinson's disease than in controls. This is the first study of Parkinson's disease to include familial cases. It confirms the absence of association between the APOE allele epsilon4 and this disease.