BACKGROUND: We assessed the effects of preventive practices on sexually transmitted infections (STIs) and reproductive tract infections (RTIs) among female sex workers (FSWs) in Lanzhou, China. METHODS: A cross-sectional survey was conducted among 350 FSWs from November 2008 to January 2009. Demographic information and behavioural factors associated with STIs were collected from all the participants. Blood samples and cervical swabs were obtained, and genital and cervical examinations were done at the collection site. A multivariate logistic regression model was used to analyze the relationships between preventive measures and the incidences of STIs and RTIs. RESULTS: The utilization rates of vaginal douching (VD), regular intravenous infusion (RII), and prophylactic oral antibiotics (POA) among FSWs were 72.9%, 35.7%, and 36.3%, respectively. The preventive measures had no impact on diagnosed STIs. On the contrary, the use of VD + POA could increase the risk of vaginal infection by 2-fold (odds ratio (OR) 2.9, 95% credible interval (95% CI) 1.3-6.7). Moreover, the risk for cervical infection increased with the use of POA alone (OR 4.0, 95% CI 1.1-15.4), VD + POA (OR 4.2, 95% CI 1.7-10.3), and VD + RII (OR 2.5, 95% CI 1.1-5.7). CONCLUSIONS: VD, RII, and POA, which were used widely by FSWs to prevent STIs after sex with their clients, were assumed to prevent STIs. Instead, our study indicates that they could increase the risk of vaginal or cervical infections. Therefore, FSWs should be informed of proper preventive strategies to reduce the incidence.