In an evaluation of 31 selected studies, statistical procedures for synthesizing data (meta-analysis) were used to assess the effects of early intervention services on disabled children younger than 3 years of age and on their families. Results indicate that early intervention is effective in promoting developmental progress in infants and toddlers with biologically based disabilities. Programs that served a heterogeneous group of children, provided a structured curriculum, and targeted their efforts on parents and children together appeared to be the most effective. Definitive evaluation of the efficacy of early intervention programs is tempered by the restricted range of outcomes measured and by a paucity of information about the children and families enrolled in such programs, as well as about the specific nature of the services received. Despite their limitations, available data provide the basis for a rational pediatric approach to early intervention programs, while highlighting specific directions for further investigation.