This experimental study evaluated the effectiveness of a research-based intervention, schema-based instruction (SBI), on students' proportional problem solving. SBI emphasizes the underlying mathematical structure of problems, uses schematic diagrams to represent information in the problem text, provides explicit problem solving and metacognitive strategy instruction, and focuses on the flexible use of multiple solution strategies. Eighty-two teachers/classrooms with a total of 1,999 seventh-grade students across 50 school districts were randomly assigned to a treatment (SBI) or control (business-as-usual) condition. An observational measure provided evidence that the SBI intervention was implemented with fidelity. Results of multilevel modeling indicated that the SBI group scored on average significantly higher than the control group on the posttest and retention test (9 weeks later) and also showed significantly more growth in proportional problem solving. There were no treatment effects on the Process and Applications subtest of the Group Mathematics Assessment and Diagnostic Evaluation. These results demonstrate that SBI can be more effective than the control approach in improving students' proportional problem solving. [This paper was published in the "Journal of Educational Psychology," (EJ1082754).]