Circular migration and injection drug use increase the risk of HIV transmission in sending communities. We describe female sex workers who are injection drug users' (FSW-IDUs) circular migration and drug use behaviors. Between 2008-2010, 258 migrant FSW-IDUs residing in Tijuana and Ciudad Juarez, Mexico responded to questionnaires. 24% of FSW-IDUs were circular migrants. HIV prevalence was 3.2% in circular migrants and 6.1% in non-circular migrants; 50% of circular and 75% of non-circular migrants were unaware of their HIV infection. Among circular migrants, 44% (n = 27) consumed illicit drugs in their birthplace; 74% of these (n = 20) injected drugs and one-half of injectors shared injection equipment in their birthplace. Women reporting active social relationships were significantly more likely to return home. Circular migrant FSW-IDUs exhibit multiple HIV risks and opportunities for bridging populations. Regular HIV testing and treatment and access to substance use services is critical for FSW-IDUs and their sexual/drug-using contacts.