The Infant Health and Development Program is an eight-site clinical trial designed to evaluate the efficacy of a comprehensive early intervention in reducing the developmental and health problems of low-birth-weight (less than or equal to 2500 g) premature (less than or equal to 37 weeks) infants. Nine hundred eighty-five infants, stratified by site and weight (less than or equal to 2000 g or 2001 to 2500 g), were randomly assigned to receive an educational curriculum focused on child development, as well as family support and pediatric follow-up, or only pediatric follow-up. At corrected age 36 months, the intervention group had significantly higher mean IQ scores than the follow-up group (mean difference in the heavier group was 13.2 and in the lighter group 6.6), significantly fewer maternally reported behavior problems, and a small, but statistically significant, increase in maternally reported minor illnesses for the lighter-birth-weight group only, with no difference in serious health conditions.