The two purposes of the pre-post naturalistic research design were to: 1) Investigate the impact of positive behavioral interventions and supports (PBIS) on the behavioral functioning of students with emotional disturbance (ED) (N = 37) served in self-contained settings; and 2) examine the extent to which teacher fidelity of PBIS implementation influenced student changes in behavioral functioning over the course of a school year. Results revealed significant reductions in externalizing and total problem behaviors for the students. Additionally, teacher fidelity to PBIS played a large and statistically significant role in improving the behavior of students with emotional disturbance. Limitations of the design and implications of the findings are discussed.