The aim of this study is to measure the effects of teaching problem-solving strategies as an integrated part of the mathematical content prescribed by the curriculum. By implementing the designs principles developed in the previous study, we carry out a one-year-long teaching intervention in a Swedish upper secondary school. Pre- and post-tests are used to compare the development of the experimental group with a control group. The results show that the development in mathematical knowledge in general is similar for the experimental group and the control group. However, the experimental group has a significantly better development in problem-solving ability. This study presents relevant implications to practitioners and other educational designers on how to enhance problem-solving ability by focusing on teaching problems-solving strategies integrated throughout the curriculum.