Background: The early diagnosis of psychiatric disorders is critical as it improves the chance of recovery for patients. The aim of this study was to determine gender disparities in psychiatric and mental disorders in adult persons and to examine the validity of the 28-item General Health Questionnaire (GHQ-28; Persian version) in the diagnosis of patients with suspected psychiatric disorders, along with receiver operating characteristic (ROC) analysis. Materials and Methods: The cross-sectional study was conducted using cluster random sampling method in three steps. Subjects were screened by GHQ-28 and then evaluated by Structured Clinical Interview for Diagnostic and Statistical Manual of Mental Disorders, 4th Edition, Text Revision (DSM-IV-TR) for diagnosis/classification of mental disorders. Chisquare test and independent t-test were used for statistical analysis. The ROC curve was used to assess cut-off points. Results: Of the 763 participants (aged 15 and above), 25.8% of responders demonstrated characteristics of psychological distress; the prevalence in males and females were 20.9% and 29.8%, respectively. The common mental disorders in males were anxiety disorder (18.2%), followed by any major depressive disorder (MDD) (17.4. %), and compulsive disorder (10.0%). In females, the common mental disorders were anxiety disorder (23.6%), followed by any MDD (22.7%), compulsive disorder (13.9%), phobia disorder (10.4%) and psychotic disorder (6.1%). ROC analysis showed that 91.7% of suspected persons had a mental disorder as assessed by DSM-IV-TR.