To determine whether the apolipoprotein E (APOE) epsilon 4 allele is a risk factor of drug-induced hallucinations in nondemented patients with Parkinson's disease (PD), the proportions of patients with hallucinations in groups with and without the APOE epsilon 4 allele were compared with a chi 2 test. The contribution of the APOE epsilon 4 allele to the occurrence of hallucinations was further evaluated by means of logistic regression models, adjusting for potential prognostic variables. Thirteen (76%) of the 17 patients who had the epsilon 4 allele had visual hallucinations, compared with 20 (23%) of the 88 patients without the epsilon 4 allele (p < 0.0001; odds ratio = 11.05; 95% CI 3.24-37.67). In addition, treatment with dopamine agonists also contributed to an increased risk of hallucinations (p = 0.0011). After adjustment for age, severity of parkinsonism, duration of treatment, dose of levodopa, and treatment with dopamine agonists, the association between the presence of the epsilon 4 allele and the occurrence of visual hallucinations remained significant (p = 0.0003). Nondemented PD patients with the APOE epsilon 4 allele have a high risk of developing drug-induced visual hallucinations. Further studies are needed to evaluate which proportion of these patients will end up developing dementia.