This paper evaluates the effectiveness of a student controlled computer program for high school mathematics based on instruction principles derived from Schoenfeld's theory of problem solving. The computer program allows students to choose problems and to make use of hints during different episodes of solving problems. Crucial episodes are: analyzing the problem, selecting appropriate mathematical knowledge, making a plan, carrying it out, and checking the answer against the question asked. The effectiveness of the computer program was evaluated by means of a pre-test-post-test quasi experimental design study. Four classes worked with the computer program in three periods of two consecutive weeks each, whereas five classes received only traditional mathematics education. These classes served as a control group. The results show evidence of intervention effectiveness. The students who worked with the computer program showed increased problem-solving ability compared to the students in traditional mathematics instruction. The use of hints could explain an essential part of the increase in students' problem solving skills.