The authors examined the potential differences between problem-based learning (PBL) and traditional instructional approaches in building knowledge of macroeconomic concepts and principles in high school students. Using data from 252 economics students at 11 high schools and controlling for individual characteristics, most notably verbal ability, they found modest evidence that, in the aggregate, PBL increased learning of macroeconomics at the high school level as compared with traditional classes. They found strong evidence of an instructional interaction with teachers such that, for some teachers, students' learning of macroeconomics increased using PBL but, for others, learning increased using more traditional instructional methods. Still other teachers saw no significant difference in learning under the two instructional strategies. The results suggest that problem-based instruction can improve student learning if instructors who are well trained in both the PBL technique and economics implement it. (Contains 10 notes.)