Three hundred and eighty six injecting drug users entered into an HIV study through methadone programs over a 40 month period. Differences in oral, intranasal and parenteral use of heroin and cocaine were assessed between four consecutive 10 month intake groups. Heroin smoking and cocaine freebasing were increased in the later intake groups, while there were no significant changes in injecting variables. No relationship between sexual risk behaviour and cocaine freebasing was found. HIV seropositivity was less among cocaine freebasers at 19% compared to 32% among non freebasers. The increase in heroin smoking and cocaine freebasing may have a favourable preventive effect on the spread of HIV among this high risk group.