The marked shift in the patterns of drug use in Iran, from opium smoking to injecting drug use, has led to serious health-related outcomes. This study was designed to explore characteristics of people who inject drugs (PWID) in Tehran, Iran. Nine hundred and four PWID were recruited from treatment and harm reduction facilities, as well as drug user hangouts in public areas in Tehran. Participants were interviewed using the Persian version of the World Health Organization Drug Injecting Study Phase II questionnaire. The median age at the time of the first illegal drug use, at the time of the first injection and current age was 20, 24 and 32, respectively. In more than 80% of the cases, the first drug used was opium. The transition from the first drug use to the first drug injection occurred after an average of 6.6 and 2.7 years for those who had started drug use with opium and heroin, respectively. Two-thirds of the participants shared injecting equipment within the last 6 months. Difficulty in obtaining sterile needles and thehigh cost of syringes were reported as the major reasons for needle/syringe sharing. Approximately 80% of community-recruited PWID reported difficulties in using treatment or harm reduction services. Self-detoxification and forced detoxification were the most common types of drug abuse treatment in alifetime. Despite a dramatic shift in drug policy in Iran during the past few years, wider coverage of harm reduction services, improvement of the quality of services, and education about such services are still necessary.