This paper estimates the effect of a state-imposed curriculum mandate on the academic achievement of US public school students. By 1998, 14 states across the US had mandates that high school students should take an economics course. For these states, the proportions of public schools students taking high school economics was around twice that of states without mandates. A curriculum mandate may be interpreted as an exogenous constraint on the production of more general academic achievement by students. Where there is a mandate to provide instruction in a particular subject, general academic test scores might be affected. Using a range of estimation techniques applied to a dataset of almost 600,000 public school students across the US, we find that students who are mandated to take economics post substantially lower scholastic aptitude test (SAT) scores. The mandate reduces test scores by around 0.08 standard deviations for those students who would not otherwise have enrolled. Such effects are not found for three other non-mandatory subjects--French, German, and biology--which in contrast convey a strong SAT premium.