The purpose of this paper is to estimate HIV seroprevalence and to examine the injection and sexual risk behaviours of a cohort of active new heroin injectors who have initiated injection within the past four years and to compare their risk behaviours with those of long-term heroin injectors who initiated injection prior to January 1, 1985. A stratified network-based sample was used to recruit injection drug users (IDUs) from the streets of Miami-Dade, Florida. New IDUs displayed a significantly lower HIV seroprevalence than long-term injectors (13.3 versus 24.7%). Both new and long-term drug injectors exhibited a high level of current HIV risk behaviour. While new injectors were more likely than long-term injectors to practise safer injection behaviours at the initial injection episode, the current risk behaviours of new and long-term injectors are similar.