Bicyclists are vulnerable road users for severe injury all over the world. The nature and extent of such injuries are less well known in Iran. Using data from a comprehensive survey conducted by the Ministry of Health and Medical Education in 13 health divisions of Iran, in 2003, we examined circumstances around bicyclist injury and death. Trained health workers completed the survey instruments by interviewing patients who stayed more than 24h in hospitals and/or relatives, hospital personnel and by reviewing patient charts. Data were cross-matched with medico-legal documents to prevent missing deaths. The information that was collected from 64 cities/towns' emergency departments (EDs), over the study period, showed that 440 injured cyclists were hospitalized and/or died due to traffic collisions. Most injuries occurred in males (94.8%) and in the young (median age: 14 years with 75% </=18 years). Head injury occurred in 14% of all hospitalized and in 90% of fatally injured bicyclists. Striking a moving vehicle increased the odds of death (OR: 32.3; 95% CI 3.5-291.0) as well as the odds of severe injury (OR: 1.9; 95% CI 1.2-3.2) compared with other mechanisms of injury. As a conclusion, bicyclists in Iran, particularly males and young children, are vulnerable to severe injury and death when struck by moving vehicles on highways.