In our study, we examine whether student achievement in the United States is affected by the share of teaching time devoted to lecture-style presentations as distinct from problem-solving activities. Employing information on in-class time use provided by a nationally representative sample of U.S. teachers in the 2003 Trends in International Mathematics and Science Study (TIMSS), we estimate the impact of teaching practices on student achievement by looking at the differential effects on the same student of two different teachers, using two different teaching strategies. We find that teaching style matters for student achievement, but in the opposite direction than anticipated by conventional wisdom: an emphasis on lecture-style presentations (rather than problem-solving activities) is associated with an increase—not a decrease—in student achievement. This result implies that a shift to problem-solving instruction is more likely to adversely affect student learning than to improve it.