We have treated 20 autistic children with behavior therapy. At intake, most of the children were severely disturbed, having symptoms indicating an extremely poor prognosis. The children were treated in separate groups, and some were treated more than once, allowing for within- and between-subject replications of treatment effects. We have employed reliable measures of generalization across situations and behaviors as well as across time (follow-up). The findings can be summarized as follows: (1) Inappropriate behaviors (self-stimulation and echolalia) decreased during treatment, and appropriate behaviors (appropriate speech, appropriate play, and social non-verbal behaviors) increased. (2) Spontaneous social interactions and the spontaneous use of language occurred about eight months into treatment for some of the children. (3) IQs and social quotients reflected improvement during treatment. (4) There were no exceptions to the improvement, however, some of the children improved more than others. (5) Follow-up measures recorded 1 to 4 yr after treatment showed that large differences between groups of children depended upon the post-treatment environment (those groups whose parents were trained to carry out behavior therapy continued to improve, while children who were institutionalized regressed). (6) A brief reinstatement of behavior therapy could temporarily re-establish some of the original therapeutic gains made by the children who were subsequently institutionalized.