The Infant Health and Development Program was an eight-site randomized controlled trial of comprehensive early intervention for low birthweight, premature infants during the first 3 years of life in which intellectual development was an outcome of major importance. At 24 and 36 months, but not at 12 months, higher Mental Development Index and IQ were associated with higher levels of participation in the intervention. In a longitudinal analysis of these data, we found that the intellectual development of children in the intervention group was associated with each of the three intervention modalities (the number of home visits received, days attended at child centers, and the number of parent meetings attended) but not with children's background characteristics (i.e., maternal education, birthweight). We suggest that these findings represent a dose-response relation between intervention and outcome.