Objective: Primary care may be an effective venue for delivering behavioral interventions for sexual safety among HIV-positive men who have sex with men (MSM); however, few studies show efficacy for such an approach. We tested the efficacy of the Treatment Advocacy Program (TAP), a 4-session, primary-care-based, individual counseling intervention led by HIV-positive MSM "peer advocates" in reducing unprotected sex with HIV-negative or unknown partners (HIV transmission risk). Method: We randomized 313 HIV-positive MSM to TAP or standard care. HIV transmission risk was assessed at baseline, 6 months, and 12 months (251 participants completed all study waves). We conducted intent-to-treat analyses using general estimating equations to test the interaction of group (TAP vs. standard care) by follow-up period. Results: At study completion, TAP participants reported greater transmission risk reduction than did those receiving standard care, [chi][squared](2, N = 249) = 6.6, p = 0.04. Transmission risk among TAP participants decreased from 34% at baseline to about 20% at both 6 and 12 months: Transmission risk ranged from 23% to 25% among comparison participants. Conclusions: TAP reduced transmission risk among HIV-positive MSM, although results are modest. Many participants and peer advocates commented favorably on the computer structure of the program. We feel that the key elements of TAP--computer-based and individually tailored session content, delivered by peers, in the primary care setting--warrant further exploration. (Contains 2 tables, 1 footnote and 4 figures.)