There is growing evidence about the importance of female sex workers (FSWs) and their clients as bridging groups for the HIV epidemic. In China the role of FSWs is key to the future trajectory of the epidemic. This study was carried out to explore factors which may increase the potential for FSWs to act as vectors for HIV transmission. Semi-structured interviews were carried out with 84 sex workers in two types of brothel in Yingjiang County of Yunnan Province, where sex work and injecting drug use are common. The key findings are that the FSWs are young, (median age 17.8 years), their engagement in sex work is short term (69% plan to work for less than one year) and the throughput of clients is low (23% only 1-2 clients per week, 71% less than seven). Awareness of HIV, STIs and condom use was generally good, but only 32% always used condoms and 18% never did. Fifty-five percent had had a health check. FSWs at the cheaper, higher throughput brothels where condom use is uncommon are a potential bridge to the general population. Interventions must focus on these FSWs and their clients to prevent them from becoming drivers of the epidemic.