Preexisting resistance can affect antiretroviral therapy efficacy in people living with HIV. One of the most common treatment-emergent resistance substitutions is M184V or, to a lesser extent, M184I. This substitution can be transmitted, archived in the viral reservoir, and reactivated, even after reversion to wild-type virus in plasma. In this pooled analysis, we investigated the prevalence of preexisting M184V/I and impact on virologic outcomes.