We examined whether parental monitoring at baseline predicted subsequent substance use in a high-risk youth population. Students in 14 alternative high schools in Washington State completed self-report surveys at three time points over the course of 2 years. Primary analyses included 1,423 students aged 14-20 who lived with at least one parent or step-parent at baseline. Using hierarchical linear modeling, we found that high parental monitoring at baseline predicted significantly less use of alcohol, marijuana, downers, cocaine, PCP, LSD, and prescription drugs and drinking to intoxication at the first posttest. Approximately 1 year later, high parental monitoring at baseline predicted significantly less use of alcohol, cocaine, prescription drugs, uppers, and ecstasy and drinking to intoxication. Study results suggest that parental monitoring serves as a protective factor, even for high-risk alternative high school students. Including a parental monitoring component may increase the effectiveness of traditional drug prevention programs.