Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved. Background: It remains unclear if methamphetamine is merely associated with high-risk behavior or if methamphetamine use causes high-risk behavior. Determining this would require a randomized controlled trial, which is clearly not ethical. A possible surrogate would be to investigate individuals before and after starting the use of methamphetamine. Methods: We performed a cohort study to analyze recent selfreported methamphetamine use and sexual risk behavior among 8905 men who have sex with men (MSM) receiving the "Early Test," a community-based HIV screening program in San Diego, CA, between April 2008 and July 2014 (total 17,272 testing encounters). Sexual risk behavior was evaluated using a previously published risk behavior score [San Diego Early Test (SDET) score] that predicts risk of HIV acquisition. Results: Methamphetamine use during the last 12 months (hereafter, recent-meth) was reported by 754/8905 unique MSM (8.5%). SDET scores were significantly higher in the 754 MSM with recentmeth use compared with the 5922 MSM who reported that they have never used methamphetamine (P < 0.001). Eighty-two repeat testers initiated methamphetamine between testing encounter, with significantly higher SDET scores after starting methamphetamine [median 5 (interquartile range, 2-7) at recent-meth versus median 3 (interquartile range, 0-5) at never-meth; P < 0.001, respectively]. Conclusions: Given the ethical impossibility of conducting a randomized controlled trial, the results presented here provide the strongest evidence yet that initiation of methamphetamine use increases sexual risk behavior among HIV-uninfected MSM. Until more effective prevention or treatment interventions are available for methamphetamine users, HIV-uninfected MSM who use methamphetamine may represent ideal candidates for alternative effective prevention interventions (ie, preexposure prophylaxis).