Adaptive social skills were assessed longitudinally at approximately ages 2, 3, 5, 9, and 13 years in a sample of 192 children with a clinical diagnosis of autism (n = 93), PDD-NOS (n = 51), or nonspectrum developmental disabilities (n = 46) at age 2. Growth curve analyses with SAS proc mixed were used to analyze social trajectories over time. Both individual characteristics and environmental resources emerged as key predictors of adaptive social behavior outcome. The gap between children with autism and the other two diagnostic groups widened with time as the social skills of the latter groups improved at a higher rate. However, within diagnostic groups, improvement ranged from minimal to very dramatic. Children with autism most at risk for problems with social adaptive abilities later in life can be identified with considerable accuracy at a very young age so they can be targeted for appropriate early intervention services.