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Background: Preexposure prophylaxis (PrEP) prevents HIV infection but relies on good adherence at times of risk, termed "prevention-effective adherence." Most studies assess adherence without reference to sexual behaviur, making it challenging to determine if poor adherence coincides with HIV risk. Setting: We examined data from a behavioral substudy of a largescale PrEP implementation trial in New South Wales, Australia. Methods: Trial participants completed optional brief quarterly surveys, reporting the number of pills taken and sexual behavior with male partners for each day of the "last full week" before each survey. Condomless sex (CLS) was defined as "higher risk" for HIV when with HIV-positive men with detectable/unknown viral loads or unknown HIV status men. Adequate PrEP protection was defined as ≥4 pills for participants assigned male sex at birth and ≥6 pills for participants assigned female sex at birth (including transgender men). Results: Of 9596 participants dispensed PrEP, 4401 completed baseline and ≥1 follow-up survey. Participants reported on 12,399 "last full weeks": 7485 weeks (60.4%) involved CLS and 2521 weeks (33.7% of CLS-weeks) involved higher risk CLS. There were 103 weeks in which participants did not have adequate PrEP protection and had higher risk CLS: 4.1% of higher-risk CLS weeks (n = 103/2521), 1.4% of all CLS weeks (n = 103/7485), and 0.8% of all observed weeks (n = 103/12,399). Conclusions: In a large PrEP trial, prevention-effective adherence to PrEP was very high at 99%. Our findings illustrate the importance of measuring pill-taking and sexual behavior in the same period so that prevention-effective adherence can be better estimated. [ABSTRACT FROM AUTHOR]