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Self-reported measures of antiretroviral adherence vary greatly in recall time periods and response tasks. To determine which time frame is most accurate, we compared 3-, 7-day, and 1-month self-reports with data from medication event monitoring system (MEMS). To determine which response task is most accurate we compared three different 1-month self-report tasks (frequency, percent, and rating) to MEMS. We analyzed 643 study visits made by 156 participants. Over-reporting (self-report minus MEMS) was significantly less for the 1-month recall period (9%) than for the 3 (17%) or 7-day (14%) periods. Over-reporting was significantly less for the 1-month rating task (3%) than for the 1-month frequency and percent tasks (both 12%). We conclude that 1-month recall periods may be more accurate than 3- or 7-day periods, and that items that ask respondents to rate their adherence may be more accurate than those that ask about frequencies or percents.