Objective: This 9 year longitudinal case study describes the successful application of a behavior modification program in a combined educational/therapeutic setting with a low functioning adolescent diagnosed with autism. The patient had no history of behavioral modification prior to entering the outpatient setting. Method: Goals were specifically set for the patient every 3 months over the 9 year period. His functional response to treatment and accomplishment of goals was assessed with the appropriate Functional Independent Measures (FIM) then classified using the International Classification of Functioning (ICF). Goals addressed receptive/expressive language, attending to social stimuli, imitation, rote counting calculations, perception, and sense of self through self-help care. Immediate and token reward systems were used to shape behavior. Domains were assessed monthly using the FIM and number of goals achieved versus those not achieved. Assessments were then classified using the ICF and analyzed using a trend analysis. Results: Data demonstrated that the patient accomplished a high percentage of goals, even reaching 85% of behavioral goals set. Improvement was also revealed by an increase from total/maximum assistance to moderate assistance in various domains over the 9 year period. Conclusion(s): Behavioral modification, in a combined special education and therapeutic outpatient program, can improve the functioning of an adolescent with severe autism. As classified by the ICF, this higher functioning covers a variety of necessary skills/domains and can be effectively tracked over time. The ICF to chart these functional improvements provides an important step in demonstrating the efficacy of long term interdisciplinary treatment especially with severe cases.