Aims. To analyse the temporal changes in demography and drug-taking behaviours of a cohort of drug users in Hong Kong . Design. An analysis of records of clients over a 5-year period . Setting. Shek Kwu Chau Treatment and Rehabilitation Centre, a large residential service offering voluntary rehabilitation programmes for male drug users in Hong Kong . Measurements. The following parameters were analysed for all admissions between 1991 and 1995: duration of drug-taking, route(s) of drug administration, type of drugs abused, practice of injection and needle-sharing, time since last needle-sharing, and expenditure on drugs . Findings. A total of 10 353 admissions were recorded in the 5-year period from 1991 to 1995, of whom 2853 (27.6%) were new admissions. The mean age was 36 years. There was a significant increase in new admission in those between the age of 13 and 19. Almost all clients were heroin users. The needle-sharing rates fell from over 20% in 1991 to 5% in 1995. Needle-sharing was more common in younger clients. The decline in the rate of needle-sharing was especially significant in those between the age of 20 and 39 . Conclusion. The proportion of young drug users in the study population in Hong Kong has risen over the 5-year period. The prevalence of high-risk drug-taking behaviours had declined, which might have contributed to the low HIV rates in the community.