Altered dopamine neurotransmission and eye movement disturbances have been implicated in the pathogenic process of schizophrenia. So far, molecular genetic studies have shown little association between schizophrenia and polymorphism of any dopamine receptor or transporter genes except for some findings concerning D3 receptor (DRD3) gene. Eye movement disturbances occur in a majority of patients with schizophrenia and in a proportion of their first-degree relatives and they have been suggested as a phenotypic marker in genetic studies of this illness. Here we report an association between the Ser9Gly polymorphism of the DRD3 gene and the intensity of eye movement disturbances (fixation and smooth pursuit) observed in 119 schizophrenic patients and in 94 unrelated healthy control subjects. In schizophrenic patients, the mean intensity of both kinds of eye movement disturbances was highest in individuals with the Ser-Ser genotype, significantly lower in Ser-Gly and lowest in the Gly-Gly genotype. The Ser-Ser genotype was more prevalent in patients with a higher intensity of both fixation (58.1 vs 23.9% P Ͻ 0.001) and smooth pursuit disturbances (52.3 vs 25.8%, P Ͻ 0.02) and the Ser-Gly genotype frequency was lower in patients with higher fixation disturbances (37.0 vs 60.9%, P Ͻ 0.02). In control subjects, the genotype frequency Ser-Ser was higher in subjects with any degree of eye movement disturbances compared to subjects without such disturbances both for fixation and smooth pursuit performance (81.0 vs 50.7%, P Ͻ 0.05 and 79.2 vs 50.0%, P Ͻ 0.05, respectively). In control subjects the frequency of Ser-Gly was lower in the first group, for either fixation or smooth pursuit, compared to normal performers (9.5 vs 43.8%, P Ͻ 0.01 and 8.3 vs 45.7, P Ͻ 0.005, respectively). We suggest that the DRD3 Ser9Gly polymorphism may be a contributing factor to the performance of eye movements used as a phenotypic marker of schizophrenia. Molecular Psychiatry (2001) 6, 718-724.The hypothesis of altered dopamine neurotransmission in schizophrenia proposed on the basis of the mechanism of antipsychotic drugs action gained some support from recent neuroimaging data. 1 However, molecular genetic studies performed in recent decades did not show a strong association between schizophrenia and any gene coding either dopamine receptor or transporter except for dopamine receptor D3 (DRD3) gene polymorphism, where the relationship was found with some aspects of this illness. DRD3 is localized in limbic and cortical brain areas and probably affects locomotion as well as emotional and cognitive processes. 2 DRD3 knock-out mice show abnormalities in spontaneous level of locomotor activity. 3 The DRD3 gene was mapped to chromosome 3q13.3 and its polymorphic site in exon 1 with a serine to glycine substitution has been postulated as a useful investigating tool in psychiatric disorder. 4 Association studies of this polymorphism with schizophrenia have been mostly negative, including the most recent work of Joober et al. 5