Association between tardive dyskinesia (TD) and Ser9Gly of the dopamine D 3 receptor (DRD 3) (Steen et al., 1997; Woo et al., 2002) and the serotonin 2C receptor (5-HT2C) genes polymorphisms has been reported in patients with schizophrenic disorder (Segman et al, 2000; Zhang et al, 2002). We investigated the presence of two polymorphisms of the promoter region of 5-HT2G receptor gene - 759C/T and -697G/C with TD as well as the association of the Ser9Gly polymorphism of the DRD 3 gene with TD. The study included 102 patients (TD: n = 52, non-TD: n = 50; matched on age, sex, years of antipsychotic medication, dose of antipsychotic medication and age of onset) diagnosed with schizophrenia according to the Diagnostic and Statistical Manual of Mental Disorders, fourth edition. All patients were unrelated Greeks. DNA was genotyped for the mentioned polymorphisms as described earlier (Lannfelt et al, 1992; Yuan et al., 2000). Deviation from the Hardy-Weinberg equilibrium was determined for all polymorphisms, apart from those of the X-linked promoter region of the 5-HT2C receptor gene, using a Pearson's χ 2 test. The prevalence of genotypes Ser9Ser, Ser9Gly and Gly9Gly in the 52 TD patients were 21% (11 of 52), 67% (35 of 52) and 12% (six of 52), respectively, P = 0.55 and q = 1 - P = 0.45. The corresponding frequencies in the 50 non-TD patients were 30% (15 of 50), 58% (29 of 50) and 12% (six of 50), respectively, with P = 0.59 and q = 0.49. Genotype frequencies from the population of TD patients were not in accordance with the Hardy-Weinberg principle, whereas those from the normal population did not deviate from this equilibrium. The prevalence of wild type and mutant -697G/C variants in the TD patients.