Aims To describe the drug use practices among female sex workers (FSWs) in Hanoi and to identify factors associated with their drug injecting. Design, setting and participants A two-stage cluster survey of 400 FSWs was conducted from June to September, 2002. Participating FSWs were both establishment- (160) and street-based (240), who were practising in seven urban and one suburban districts of Hanoi. Measurements Subjects were interviewed face to face using a structured questionnaire. Findings Among the middle-class FSWs, 27% used drugs, of whom 79% injected. Among low-class FSWs, 46% used drugs and 85% injected. Among drug-using FSWs, 86% had started using drugs within the past 6 years. Among drug-injecting FSWs, 81% had started injecting within the past 4 years. Cleaning of injecting equipment was not common among those who shared. Having drug-injecting 'love mates', drug-using clients, longer residence in Hanoi, more clients and not currently cohabiting were found to be independently associated with drug injecting among FSWs. Conclusions The high prevalence of injecting drug use among FSWs makes them susceptible to HIV infection, and is a threat to their clients. There is a strong relationship between drug-using FSWs and male drug-using clients and non-client partners. Intervention to prevent drug use initiation among non-drug-using FSWs and harm reduction among drug-using FSWs are urgently needed.