To assess the effectiveness of a peer-based intervention on adherence to and completion of latent tuberculous infection (LTBI) treatment. Patients prescribed self-administered LTBI treatment were enrolled in a randomized controlled trial of an experimental, peer-based adherence support intervention. Primary outcomes were treatment adherence and completion. Adherence was assessed through self-report, electronic monitoring devices and clinic visits. Of 250 participants, 70% were male; 71% were Black and 20% Latino; the mean age was 40 years; 67% were foreign-born and 39% were married. No significant baseline differences were noted between the intervention groups. Treatment completion was 61% in the intervention group compared to 57% in the controls (P = 0.482). The corresponding completion rate for other clinic patients was 44%. Foreign birth, marriage and history of mental illness were associated with non-completion of treatment after controlling for the intervention group; increased completion rates were found among foreign-born married persons and older participants. A substantial difference in adherence rates was observed between the intervention groups. Adherence among non-completers decreased early, while adherence among completers remained constant. The peer-based intervention was not significantly associated with LTBI treatment completion, but was associated with greater adherence. Findings suggest the importance of interventions to support adherence that target early non-adherence with LTBI treatment, particularly in the first 2 months, when there is a substantial risk of default.