Long hours of paid employment during high school have been linked to a variety of problem behaviors, but questions remain about whether and to what extent work intensity makes any causal contribution. This study addresses those questions by focusing on how 12th-grade work intensity is associated with substance use and educational attainment in the years following high school. It uses 2 nationally representative longitudinal data sets from the Monitoring the Future project, spanning a total of 3 decades. One data set tracks 8th graders for 8 years (modal ages 14-22) and provides extensive controls for possible prior causes; the second, larger data set tracks 12th graders for up to 12 years (to modal ages 29-30) and permits assessment of possible short-term and longer term consequences. Findings based on propensity score matching and multivariate regression analyses are highly consistent across the 2 sets of data. All findings show that more fundamental prior problems, including low academic performance and aspirations, make substantial contributions to substance use and long-term academic attainment (selection effects), but the findings also suggest that high work intensity during high school has long-term costs in terms of college completion and perhaps cigarette use. (Contains 1 figure, 6 tables, and 12 footnotes.)