CHOICES was an open-label, randomized, comparative effectiveness trial of office-based extended-release naltrexone versus treatment as usual in people with untreated opioid use disorder and HIV. This study explored facilitators to recruitment in Miami, a successful recruiting site in the national trial. The mixed-methods study included quantitative surveys of randomized participants, medical record abstraction, and qualitative interviews with study staff. Miami recruited 47 (40.5%) of 116 randomized participants in the six-site national trial. In-depth interviews of study staff (n = 6) revealed that Miami had a recruitment approach consisting of street level outreach and a close relationship with the local syringe services program (SSP). Partnership with a local SSP provided access to people living with HIV who inject drugs in Miami. SSPs’ fundamental trust within the community of people who inject drugs can be leveraged in studies aiming to improve health outcomes in this underserved and high-priority population.