The relation between fidelity of implementation and student outcomes in a computer-based middle school mathematics curriculum was measured empirically. Participants included 485 students and 23 teachers from 11 public middle schools across seven states. Implementation fidelity was defined using two constructs: fidelity to structure and fidelity to process. Because of the nested nature of the data, we used a two-level hierarchical linear model for analysis. Four variables, all categorized as fidelity to structure variables, proved significant—total time in intervention (p <.001), concentration of time in intervention (p = .03), direct observation of intervention fidelity (p = .04), and pretest score (p <.001). Fidelity to process was found to be nonsignificant. The importance of measuring the relation between implementation fidelity and student outcomes is discussed as well as implications for researchers and teachers.