Association of the dopamine D3 receptor gene (DRD3) and schizophrenia was examined in unrelated Israeli and Italian schizophrenic patients and ethnically matched normal control subjects. In the combined sample, there was a siginificant excess of DRD3 allele 2 among the schizophrenic patients (χ2 = 4.70, d.f. 1, p = 0.03). Comparison of genotype frequencies revealed an excess of the 2-2 genotype in the combined schizophrenic sample (χ2 = 8.30, d.f. 1, p = 0.01) and in the non-Ashkenazi Israeli schizophrenics alone (χ2 = 5.70, d.f. 2, p = 0.05). DRD3 2-2 genotype conferred a significantly increased risk of schizophrenia (χ2 = 8.21, d.f. 1, p = 0.004; OR = 2.87, CI 95% = 1.36-5.76) in the combined sample and in the non-Ashkenazi Israeli schizophrenics (χ2 = 7.22, d.f. 1, p = 0.04; OR = 7.22, CI 95% = 1.04-24.83). In the combined and Italian samples, allele 2 was associated with early age of onset as was the 2-2 genotype in the combined sample and non-Ash-kenazi group. The 2-2 genotype was associated with poor response to neuroleptics, particularly in the non-Ashkenazi, Israeli schizophrenics. The possibility that DRD3 or a locus in linkage disequilibrium with it may play a role in the transmission of schizophrenia, is considered in relation to previous positive and negative rep