Although Early and Intensive Behavioral Intervention (EIBI) is an effective treatment for many children with autism, there is a substantial individual difference in outcome. This study was designed to investigate whether treatment gains were associated with the number and type of stimuli that function as reinforcers for 21 preschool-aged children with autism. Children with a large repertoire of socially mediated reinforcers benefited more from treatment. Children with many stereotypic behaviors, assumed to be an effect of a larger repertoire of automatic reinforcers, exhibited less benefit from treatment. These two dimensions taken together explained 49.9% of the variation in treatment gains for children after one year of EIBI. Due to the retrospective and indirect design of the study, results are to be interpreted with caution. (Contains 1 figure.)