We report the results of a controlled evaluation of an interactive on-line tutoring system for high school math achievement test problem solving. High school students (N = 202) completed a math pre-test and were then assigned by teachers to receive interactive on-line multimedia tutoring or their regular classroom instruction. The on-line tutored students improved on the post-test, but the effect was limited to problems involving skills tutored in the on-line system (within-group control). Control group students showed no improvement. Students' use of interactive multimedia hints predicted pre- to post-test improvement, and benefits of tutoring were greatest for students with weakest initial math skills.