Promoting the self-determination of adolescents with disabilities has become best practice in secondary education and transition services, but to date there have been no studies establishing a causal relationship between efforts to promote self-determination and enhancement of the self-determination of youth with disabilities. This article reports a randomized trial placebo control group study of 371 high school students receiving special education services under the categorical areas of mental retardation or learning disabilities. Students were randomly assigned to an intervention or control group (by high school campus), with students in the intervention condition receiving multiple instructional components to promote self-determination. Latent growth curve analysis showed that although all students in the study showed improved self-determination over the 3 years of the study, students in the intervention group showed significantly greater growth, though specific intraindividual variables affected this growth. Implications for research and intervention are discussed. (Contains 5 tables and 2 figures.)