Initiation into injecting is a crucial event for continued reproduction of an injecting drug using (IDU) population and for exposure to blood-borne viruses, but little is known about how this happens. Three hundred young injectors were interviewed in Melbourne by peer workers within the first few years of beginning to inject, about the circumstances surrounding their initiation. Most had indications of social disruption, including having left school early, unemployment, family disruption, homelessness and incarceration. First drug injected was most often amphetamines (average age 16 years), most having already used amphetamines by a different route of administration, but with a steady movement thereafter to heroin as the drug of choice. The most common scenario was one in which injecting was unplanned but the person was active in bringing about the initiation. Most identified a significant other who initiated them (few of whom were dealers), and over half had subsequently initiated others into injecting, on average 0.6 per year; after 5 years 237 young injectors had initiated at least 420 others. Those who initiated multiple others were more likely to be unemployed, to inject multiple drugs and to have dealt. Modelling injecting as a communicable phenomenon, where appropriate, may help estimate population dynamics among IDUs. Peer education programmes are likely to be the most effective harm reduction approach among new injectors.