In 1991 we reported on differences between younger and older injecting drug users (IDUs) in the Australian National AIDS and Injecting Drug Use Study. In 1994, a second large multi-city study of IDUs, the Australian Study of HIV and Injecting Drug Use (ASHIDU) allowed a repetition of that analysis to see whether age differences demonstrated in 1991 could still be found. Using discriminant function analysis, we found that younger (under 23) IDUs were more likely than older ID Us to be female, to have used hallucinogens in the previous month and to have used condoms during the last sexual encounter; and less likely to be in drug treatment. Young IDUs were more mobile, injected in larger groups, had had fewer HIV/AIDS tests, and used tranquillizers less frequently than older IDUs. These differences are similar in some respects to those found in 1991, but there were no differences in needle sharing as were found in 1991. The social context of younger IDUs' lives and drug use, and the difficulty in accessing them through drug treatment agencies are noted as factors to be taken into consideration when designing harm reduction strategies for this group.