Background: Methadone maintenance treatment (MMT) can theoretically reduce the psychosocial harms of opioid dependency. The increasing number of MMT clinics in Iran during the last two decades was not accompanied by a reduction in addiction rates. Therefore, this study was conducted to compare the psychopathological characteristics of individuals on MMT, opioid dependents, and a control group in the Iranian population. Methods: This cross-sectional study was conducted in Mashhad to evaluate the psychopathological profile of 99 participants (33 opioid-dependent individuals [ODI], 33 MMT clients, and 33 controls) using the Symptom Checklist-90-Revised (SCL-90-R) questionnaire and demographic form. Parametric and non-parametric tests were used to compare the mean score of symptoms between the three groups. The Spearman correlation test was used to test the correlation of psychological features with age and dependency duration. Findings: There was no significant difference between MMT clients and ODI regarding the Global Severity Index (GSI) and nine items of the SCL-90-R questionnaire. However, a significant difference was found between the controls and two other groups (P<0.001). Paranoid ideation in the ODI was more frequent compared to MMT clients significantly (P=0.015). Psychological characteristics had a direct correlation with dependency duration and an inverse correlation with age (P<0.05). Conclusion: Psychopathologic features observed in the dependent patients and MMT clients were significantly higher than in the control group. It seems that the psychopathology profile in MMT clients was not better than dependent individuals in Iran.