The present study examines the efficacy and durability of the PEERS Program, a parent-assisted social skills group intervention for high-functioning adolescents with ASD. Results indicate that teens receiving PEERS significantly improved their social skills knowledge, social responsiveness, and overall social skills in the areas of social communication, social cognition, social awareness, social motivation, assertion, cooperation, and responsibility, while decreasing autistic mannerisms and increasing the frequency of peer interactions. Independent teacher ratings revealed significant improvement in social skills and assertion from pre-test to follow-up assessment. Examination of durability of improvement revealed maintenance of gains in nearly all domains with additional treatment gains at a 14-week follow-up assessment.