This study examined the relationship between the implementation of comprehensive school reform (CSR) and changes in reading and math achievement from 1999 until 2003. Survey data about CSR implementation and school-level achievement data were collected for multiple years from a sample of CSR schools and compared with a sample of matched comparison schools. The sampled CSR schools adopted several promising CSR models. Results revealed that improvement over the years in school-level mathematics and reading achievement of CSR schools was not consistently greater than the improvement for comparison schools. The relationship between implementation of CSR and improvement in student achievement was shown to vary by number of years, level of implementation, and CSR designation. CSR schools experienced the largest increases in student achievement between Years 3 and 5 of implementation. A high level of implementation and comprehensiveness of implementation was weakly associated with a large increase in mathematics and reading achievement relative to the matched comparison school. This relationship also varies by CSR designation.