No DAT deterrent effects were found for past-month drug/alcohol use during any of four followup periods. Prior-year drug use was reduced for DAT athletes in two of four followup self-reports, and a combination of drug and alcohol use was also reduced at two assessments. Paradoxically, across all assessments DAT athletes reported less athletic competence, less belief that authorities were opposed to drug use, and greater risk-taking. At the final assessment, DAT athletes believed less in testing benefits and less that testing was a reason not to use drugs compared with athletes at non-DAT schools. More research is required before DAT can be considered an effective deterrent for substance use by student-athletes. This was a 2-year prospective randomized controlled study of a single cohort among five DAT intervention high schools and six schools with a deferred DAT policy. DAT athletes (n=777) were subject to random testing during the entire academic year. Positive test results were reported to parent/guardians, with mandatory counseling. Measures of illicit drug use, with and without alcohol use, were assessed at the beginning and end of each school year, for the past month, and for the prior year. Assessments were done with voluntary, confidential questionnaires. 1 figure, 5 tables, and 41 references