In the Fall of 2004, 1,948 5th grade students from Phoenix, AZ enrolled in an evaluation of a school-based, substance use prevention intervention. To assess the consistency of Mexican and Mexican-American students' self-reports of lifetime substance use, the present study analyzed data reported by 1,418 students who reported Mexican ancestry and completed 2 to 6 questionnaires administered over a 40-month period. By wave 6, which was completed in March 2008, lifetime alcohol, cigarette, marijuana, and inhalant use rates were 86.0%, 65.0%, 64.5%, and 62.1%, respectively. Corresponding rescission rates were 24.0%, 9.6%, 5.8%, and 9.2%. Reporting patterns with one "Yes-No" sequence accounted for more than 88% of the inconsistent self-reports. This finding suggests that the majority of Mexican/Mexican-American preadolescents participating in a substance use prevention intervention provided logically consistent self-reports of lifetime substance use. (Contains 4 tables.)