Promoting self-determination has become a best practice in special education. There remains, however, a paucity of causal evidence for interventions to promote self-determination. This article presents the results of a group-randomized, modified equivalent control group design study of the efficacy of the Self-Determined Learning Model of Instruction (SDLMI, Wehmeyer, Palmer, Agran, Mithaug, & Martin, 2000) to promote self-determination. The authors used data on self-determination using multiple measures collected with 312 high school students with cognitive disabilities in both a control and a treatment group to examine the relationship between the SDLMI and self-determination. After determining strong measurement invariance for each latent construct, they found significant differences in latent means across measurement occasions and differential effects attributable to the SDLMI. This was true across disability category, though there was variance across disability populations. (Contains 4 tables and 2 figures.)