Some patients with idiopathic Parkinson's disease experience hallucinations as a result of treatment with levodopa and dopamine agonists. There is evidence for some heterogeneity in these hallucinating patients based on duration of Parkinson's disease at onset of hallucinations. We compared the frequency of polymorphisms in the dopamine D2 and D3 receptor genes between patients with drug-induced hallucinations and non-hallucinating patients. Two polymorphisms close to DRD2 and one in DRD3 were studied. No association was found with the whole group of hallucinating patients and their controls. However, an association was found with late-onset hallucinations and the C allele of the TaqIA polymorphism, 10.5 kb 3' to DRD2. This polymorphism may be in linkage disequilibrium with a mutation in DRD2 or a nearby gene that predisposes to drug-induced hallucinations which occur later in the course of idiopathic Parkinson's disease.