Data from 1433 children and adolescents with autism spectrum disorders (ASD) participating in the Simons Simplex Collection were examined to (1) investigate change in social-communication symptoms, and (2) examine predictors of improvement, particularly community-based treatments. Measures included the "Autism Diagnostic Interview--Revised" ("ADI-R"), IQ, demographics, and parent-reported treatment history (occupational, speech, and behavioral therapy). Baseline (age 4-5) and current social-communication symptom severity scores were calculated using 15 "ADI-R" items. The majority (95.4%) demonstrated improvement (between age 4-5 and current), 2.2% had no change, and 2.4% worsened. Controlling for age and initial severity, those who had received therapy had the best outcomes, greater therapy intensity predicted improvement, and the response to therapy was greatest among those with higher nonverbal IQ. (Contains 7 tables and 1 figure.)