We conducted a secondary data analysis of 11 AIDS Clinical Trials Group (ACTG) studies to examine longitudinal associations between 14 self-reported antiretroviral therapy (ART) adherence barriers (at 12 weeks) and plasma HIV RNA (at 24 weeks) and to discern the relative importance of these barriers in explaining virologic detectability. Studies enrolled from 1997 to 2003 and concluded between 2002 and 2012. We included 1496 (54.2% of the original sample) with complete data. The most commonly selected barriers were "away from home" (21.9%), "simply forgot" (19.6%), "change in daily routine" (19.5%), and "fell asleep/slept through dosing time" (18.9%). In bivariate analyses, "too many pills to take" (OR=0.43, p<0.001), "wanted to avoid side effects" (OR=0.54, p=0.001), "felt drug was toxic/harmful" (OR=0.44, p<0.001), "felt sick or ill" (OR=0.49, p<0.001), "felt depressed/overwhelmed" (OR=0.58, p=0.004), and "problem taking pills at specified time" (OR=0.71, p=0.04) were associated with a lower odds of an undetectable HIV RNA. "Too many pills to take," "wanted to avoid side effects," "felt drug was toxic/harmful," "felt sick/ill,", and "felt depressed/overwhelmed" had the highest relative importance in explaining virologic detectability. "Simply forgot" was not associated with HIV RNA (OR=0.99, p=0.95) and was ninth in its relative importance. Adherence interventions should prioritize barriers with highest importance in explaining virologic outcomes rather than focusing on more commonly reported barriers.