We used antiretroviral (ARV) drug testing to evaluate the accuracy of self-reported data for HIV status and antiretroviral treatment (ART) among people who inject drugs enrolled in an HIV prevention trial. ARV drugs were detected in enrollment samples from 72/482 = 14.9% HIV-infected participants (39/52 = 75.0% who reported being on ART; 33/430 = 7.7% who reported not being on ART). Overall, 213/482 = 44.2% participants indicated that they were not aware of their HIV-positive status prior to study entry; of those, 30 had ARV drugs detected at enrollment, including 15 who also had ARV drugs detected at the screening visit. These participants were likely aware of their HIV-positive status at study entry but did not report this to study staff. This study shows that self-reported data on HIV testing history and ART may not be accurate and that ARV drug testing can help identify persons who are aware of their HIV-positive status and are on ART.