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Background: We sought to identify factors associated with condom use during anal intercourse among self-identified human immunodeficiency virus-negative gay, bisexual, and other men who have sex with men (GBM) in Vancouver, Canada following "treatment as prevention" (TasP) scale-up in 2010.Methods: Sexually active GBM were recruited using respondent-driven sampling from 2012 to 2014. We analyzed participants' most recent sexual encounter with up to their last 5 sexual partners within the past 6 months. In addition to individual- and event-level explanatory factors, we assessed potential associations with TasP awareness, TasP-related prevention practice (viral load sorting), and TasP-related attitudes (human immunodeficiency virus treatment optimism). Accounting for clustering at the respondent-driven sampling chain-level and participant-level, factors associated with event-level condom use versus nonuse were determined using a multivariable generalized linear mixed model built using backward selection and AIC minimization.Results: Of 513 participants, 436 GBM (85%) reported a total of 1196 anal sex events with 56% condom use. The proportion of condom-protected sexual events decreased monthly over the study period (odds ratio [OR], 0.95 per month, 95% confidence interval [CI], 0.92-0.98). The TasP practices and attitudes were significantly associated with lower odds of condom use at the univariate level, but were no longer significant at multivariate level. In the multivariable model, event-level partner methamphetamine use (adjusted OR [aOR], 0.18; 95% CI, 0.06-0.58), frequency of recent anal intercourse with that partner (aOR, 0.97 per act; 95% CI, 0.95-0.98) and time since first sex with that partner (aOR, 0.97 per 6 months; 95% CI, 0.95-0.99) were associated with lower odds of condom use, whereas event-level participant alcohol use (aOR, 1.41; 95% CI, 1.01-1.98) and no planned future sex with that partner (aOR, 1.56; 95% CI, 1.08-2.27) were associated with greater odds of condom use. Event-level receptive-only (aOR, 2.10; 95% CI, 1.38-3.20) or insertive-only (aOR, 2.53; 95% CI, 1.64-3.90) sexual positions were associated with greater odds of condom use compared with reporting both positions.Conclusions: The TasP-related factors were not the most salient predictors of GBM's condom use. Health promotion must consider associations between condomless anal sex and substance use and relational factors. [ABSTRACT FROM AUTHOR]