Using cost-of-illness methodology applied to a comprehensive survey of 114 daily opiate users not currently in or seeking treatment for their addiction, we estimated the 1996 social costs of untreated opioid dependence in Toronto (Ontario, Canada). The survey collected data on social and demographic characteristics, drug use history, physical and mental health status, the use of health care and substance treatment services, drug use modality and sex-related risks of infectious diseases, sources of income, as well as criminality and involvement with the law enforcement system. The annual social cost generated by this sample, calculated at Canadian $5.086 million, is explained mostly by crime victimization (44.6%) and law enforcement (42.4%), followed by productivity losses (7.0%) and the utilization of health care (6.1%). Applying the $13,100 cost to the estimated 8,000 to 13,000 users and 2.456 million residents living in Toronto yields a range of social cost between $43 and $69 per capita.