A semi-Markov model was used to estimate final costs and outcomes. The model (no memory) was based on five states (HIV negative, HIV positive asymptomatic, HIV positive symptomatic, AIDS, Death). In the model 10% of the general population was recruited into the IVDU population, while 2.5% departed yearly for non-fatal causes. Three risk profile groups were modelled: (1) non-sharers of needles; (2) low frequency sharers; (3) high frequency sharers.