To assess changes in risk behaviours for transmission of bloodborne viruses and incidences and prevalences of these viruses in a field-recruited cohort of injecting drug users.
Prospective longitudinal cohort study.
Metropolitan and rural Victoria, June 1990 to December 1995.
626 current injecting drug users (i.e., who had injected drugs within the previous 12 months).
Seroconversion to HIV and hepatitis B and C viruses (HBV and HCV), risk behaviours for infection.
HIV incidence was very low (0.2 per 100 person-years). HCV incidence was high (10.7 per 100 person-years), but fell throughout the study, although the downward trend did not reach statistical significance. HBV incidence was moderate (1.8 per 100 person-years) and did not fall. Prevalence of risk behaviours, notably sharing needles and syringes, decreased significantly.
Significant change has occurred in the risk behaviours of the cohort, confirming results of cross-sectional studies of injecting drug use. This change may be responsible for the apparent decline in HIV and HCV incidence. Further studies are needed to monitor the incidence of bloodborne viruses in injecting drug users. Efforts to decrease risk of transmission should continue.