High adherence and maintenance of blinding are critical for placebo-controlled efficacy trials of HIV-1 biomedical prevention strategies. We assessed adherence to study drug and factors affecting adherence, including perceived randomization group, in a post-trial questionnaire of participants who completed HPTN 039, a randomized, placebo-controlled trial of HSV-2 suppression with twice-daily acyclovir to reduce HIV-1 acquisition. Of the 3172 trial participants, 2003 (63%) completed the post-trial questionnaire. Of these 2003, 72% reported missing a dose of study drug less than twice a week. Study drug adherence was not compromised by perceived randomization or genital ulcer symptoms during the study. Alcohol use was cited as an adherence barrier in some populations. Assessment of study drug adherence during and at the end of trials can evaluate perceptions of randomization and adherence by randomization arm, help to better understand barriers to and motivations for adherence, and develop interventions to increase adherence for future trials. © Springer Science+Business Media, LLC 2010.