Indices of classification accuracy of the Substance Use/Abuse scale of a Spanish-language version of the Problem Oriented Screening Instrument for Teenagers (POSIT) were evaluated among school-based youth in Mexico. Participants were 1203 youth attending one middle school (N = 619) and one high school (N = 584) in the third largest city of Coahuila, a northern border state in Mexico in May 1998. More than 94% of youth enrolled in the participating middle school and 89% of youth enrolled in the participating high school completed the International Longitudinal Survey of Adolescent Health. Indices of classification accuracy of the POSIT Substance Use/Abuse scale were evaluated against a "drug abuse" problem severity criterion that combined youth meeting DSM-IV criteria for alcohol abuse/dependence disorders with youth having used other illicit drugs five or more times in their lifetime. The present study findings suggest that using a cut score of one or two on the POSIT Substance Use/Abuse scale generally yields optimal classification accuracy indices that vary somewhat by gender and school subgroups. Further, classification accuracy indices of the POSIT Substance Use/Abuse scale are slightly better when used among high school males due, in part, to the higher base rate of serious involvement among this group compared to others.