Objectives: This survey was implemented as a part of the Urban Health Equity Assessment and Response Tool “Urban HEART Project” in Tehran, Iran. Since an accurate programming in order to provide mental health services in the province of Tehran requires basic and essential information, the present study was carried out to determine the mental health status of a population sample aged 15 years and over in the mentioned city. Method: The present research was a descriptive cross-sectional study. The statistical population comprised all individuals residing in the 22 areas of Tehran Municipality. Regular random cluster sampling was used in order to choose the subjects from among the households in the mentioned areas. In total, 19370 subjects (7705 male and 11665 female) completed the 28-item version of the General Health Questionnaire for the assessment of mental health. Data were collected and analyzed by trained personnel of Tehran municipality health centers. Results:  The results of this survey indicated that 34.2% of the population in the age group 15 years and above suffer from mental disorders (37.9% of women and 28.6% of men). Women’s relative risk of developing mental disorders was 1.3 times higher than that of men. The risk of mental disorders increased with age and divorced or widowed people were 1.5 times more likely to develop mental disorders compared with single and married individuals. Housewives and unemployed men had the highest risk of developing mental disorders. The highest rates of positive responses to questions on the GHQ-28 were related to malaise and being bad-tempered, anxiety and apprehension, inability to allocate time to tasks, insomnia and hopelessness. Anxiety and somatization were more common than social dysfunction and depression. Conclusion: It is estimated that about 2 million individuals in Tehran require mental health care. Also, insufficient number of psychiatrists and psychiatric beds, indicates that there is a need to provide appropriate staff and facilities to make mental health care available in large cities such as Tehran. The provision of life skills workshops for general population is also suggested, in order to enable individuals to cope with stressful situations in urban areas.