Background: The angiotensin converting enzyme (ACE) gene, which has been found to have an insertion and deletion polymorphism (I/D), is of increasing interest in etiology and treatment of various psychiatric disorders such as panic disorder. The present study aimed to investigate the relationship between ACE polymorphism and panic disorder., Materials and Methods: In this study, 43 patients diagnosed with panic disorder at the Erenköy Mental and Neurological Diseases Training and Research Hospital, Istanbul and 41 healthy controls were enrolled. The ACE gene insertion/deletion polymorphism of exon 16 was evaluated using the polymerase chain reaction method., Results: There was a significant association between I/D genotype and panic disorder (p=0.003). However, the frequency of the I allele was found to be significantly higher in patients compared to controls (p=0.002). In addition, we recognized a significant association between I/D polymorphism and respiratory-type panic disorder in patients. Carriers of the D allele also had an increased risk of respiratory type panic disorder patients (p=0.034). Moreover, the result of Spearman correlation analysis showed an association with ACE D allele and severity of panic disorder (p<0.001)., Conclusion: We suggest that the I/D polymorphism of the ACE gene is associated with panic disorder and particularly respiratory-type panic disorder in patients. The I/D polymorphism of the ACE gene seems to influence therapeutic outcome in patients suffering from panic disorder. Our results indicate that ACE D allele is associated with the severity of panic disorder., (Copyright © 2014 International Institute of Anticancer Research (Dr. John G. Delinassios), All rights reserved.)