This study examined the efficacy, social validity, and durability of a positive behavior support (PBS) approach with the family of a girl with autism and severe problem behavior. The study was conducted across a 10-year period beginning when the child was 5 years old. A multiple baseline across family routines design evaluated the functional relationship between parent implementation of a PBS plan and longitudinal improvements in child behavior and successful participation in routines. Daily indicator behavior data allowed us to assess generalized improvements in child behavior. An inventory of monthly community activities allowed us to assess changes in child quality of life. In addition, social validity and contextual fit were assessed. Results document that the intervention was associated with a 75% reduction in problem behavior, and that the effects were maintained across a 6-month to 7-year follow-up period. Associated outcomes included generalized improvements in child behavior and enhanced community activity patterns. Parents also rated the social validity and contextual fit of the approach highly. Results verify the efficacy and social validity of the approach and offer preliminary descriptive evidence of its durability. Contributions to the literature, implications, and future directions are discussed. (Contains 3 tables and 4 figures.)