Aim: The insertion/deletion (I/D) polymorphism of the angiotensin-converting enzyme (ACE) gene is of increasing interest in etiology and treatment of various neuropsychiatric disorders. The present study aimed at detecting the incidence of ACE gene I/D polymorphism in patients with schizophrenia living in the Eskisehir region (Turkey) and also at determining whether this illness could be associated to ACE gene I/D polymorphism and serum ACE concentrations. Methods: In our study, genomic DNA was studied in a total of 237 individuals, 132 of them having been diagnosed as patients with schizophrenia and 105 of them being used as control subjects. In addition, sera from 31 patients with schizophrenia and 26 healthy subjects were used to compare serum ACE concentrations. By using polymerase chain reaction, we determined the frequency of ACE gene I/D polymorphism and measured the serum ACE concentrations by ELISA. Results and Conclusion: Distribution of ACE gene I/D polymorphism and allele frequencies between the control group genotype proportions (II 19%, ID 44%, DD 37%) and the patient group (II 19%, ID 45%, DD 36%) were not significantly different. Serum ACE concentrations were 293.15 +/- 23.29 ng/mL in the control group and 362.61 +/- 19.96 ng/mL in the patients. It was observed that serum ACE concentrations significantly increased in patients with schizophrenia compared with those of the control group (p < 0.05). However, no significant difference could be observed according to genotypes in serum ACE concentration.