In order to determine the rate of mental disorders in male prisoners, 120 men ages 18 to 28 incarcerated in Ghasr Prison of Tehran for theft, assult, addiction, murder, unlawful sexual acts or embezzlement were selected randomly. 120 men in the same age range who were matched with the first group in terms of socio-economic characteristics were selected from normal population as a comparison group. None of the men in the latter group had any criminal records. Data were collected through SCL-90-R and Personal Characteristics Questionnaire and were analyzed using t-test. Results showed that the rate and the severity of mental disorders in all studied 9 categories of disorders, that is phobia, anxiety, somatization, psychotism, depression, paranoid thoughts, aggression, interpersonal sensitivity and obsession-compulsion were significantly higher among the prisoners than the normal group. The level of severity was higher in paranoid thoughts, obsessive compulsive disorder, and interpersonal sensitivity than in other disorders.