This paper examines if and how temporary migration increases the risk for HIV/STDs in China. Results suggest that temporary migrants had significantly higher prevalence of HIV-risk sexual and drug using behaviours but no significant differences were found between migrants and non-migrants in prevalence of HIV/STDs. Employing logistic regression analysis, we examined three mechanisms--lax social control, social isolation and migrant selectivity--by which the process of migration may lead to behaviour changes that increase migrants' HIV/STD risks. Results indicate that post-migration lax social control was the most significant mediating factor between migration and HIV risk behaviours. Temporary migrants are at high risk of HIV/STDs. Prevention interventions need to pay particular attention to migrants' post-migration lax social control.