Although seat-belts are the most effective means of preventing severe injuries and deaths from road accidents, drivers and passengers in many developing countries neglect to wear them. This study aimed to measure the rate of use of seat-belts and factors affecting their use in Tehran, Islamic Republic of Iran. In direct observations of 7718 vehicles at randomly selected sites in Tehran, 77.9% of drivers and 43.7% of front passenger had seat-belts fastened. More women used seat-belts than did men while driving but fewer as front-seat passengers. Rates of seat-belt wearing among both drivers and front passengers of vans and pick-up vehicles was low. In a separate interview study of 666 citizens, 62.4% stated that they always wore seat-belts while driving. The most frequently reported reasons for using seat-belts were avoiding fines, fear of injuries and respecting the law. To improve seat-belt use, it is recommended that law enforcement accompanied by public education should be a priority.