Parkinson's disease (PD) patients often develop dementia, and Alzheimer's disease (AD) patients frequently develop parkinsonian signs. The apolipoprotein E ε4 allele is associated with increased risk and earlier onset of AD. We studied 137 unrelated white PD patients. Those with ε4 had the earliest onset (52.7 ± 9.8 years), ε3/ε3 patients had an intermediate onset (56.1 ± 11.1 years), and those with ε2 had the latest onset (59.1 ± 13.4 years). The age at onset distribution for ε4/ε- was significantly earlier than for ε3/ε3 and ε2/ε3. These preliminary results suggest that apolipoprotein E genotypes modulate the age at onset of PD.