The main drug of choice in China is heroin. According to a report of the National Narcotic Control Commission (NNCC), 87.6% of drug users abused heroin in 2002 [12] . The abuse of ATS and MDMA (methylenedioxymethamphetamine or ecstasy) has become popular in city night clubs in recent years [1] . In the 1980s, farmers living in rural bordering areas in Yunnan and Guangxi provinces constituted a large fraction of drug users. Since the early 1990s, more and more urban residents use illicit drugs. The majority of drug abusers are young people with a low education level and limited job skills, although a small proportion of urban users regard using drugs as an indicator of "high social class." NNCC data showed that 74% were aged 17-35 years in 2002 [12] . Experimentation (90%), peer pressure (44%), and relaxation (42%) are commonly cited reasons for beginning to use drugs. Initially, drugs were taken primarily through sniffing/snorting (55%) or smoking cigarettes mixed with drugs (38%) [13] . Injection of drugs became increasingly common among drug users, probably as a result of increasing prices of illicit drugs and greater cost effectiveness of injecting to achieve the desired effect. National behavioral surveillance data showed that the median prevalence of IDU among drug users increased from 35% in April 1995 to 49% in April 2004, and median prevalence of needle sharing among IDUs also increased from 26% to 43% during this period [14, 15] . "Buying from drug or grocery stores" and "buying from hospitals" were the two most common routes for obtaining injection needles and syringes [16] . Reasons for sharing a needle included: "do not care about anything else when hooked," "thought it had been cleaned up," "difficult to buy or obtain," "only sharing with selected persons," "no money to buy," and "following other drug users" [16] . Besides IDU and needle sharing, other risk behaviors including unprotected commercial sex put drug users and ultimately non drug users at high risk of HIV [17, 18] . It is believed that many female drug users sell sex for drugs, and limited published research also provides supporting evidence [18] [19] [20] . However, the impact of the interaction between IDU and commercial sex on HIV risk is not clear. Considering the dramatic increase in the commercial sex industry in China, and the potential bridging role of those with dual risk behaviors in transmitting HIV from high risk groups to the general population, research on the interaction of IDU and commercial sex should be given high priority.