Effects of high participation in the Infant Health and Development Program (IHDP), an 8-site randomized trial that targeted low-birth-weight (LBW) premature infants (N=1,082), were estimated. Children in the treatment group were offered high-quality center-based care in their 2nd and 3rd years of life (full-day care, 50 weeks per year). High-dosage effects were estimated with a new methodology that found a matched comparison group within the follow-up group for those with high participation rates; these estimates were compared with traditional intention-to-treat (ITT) estimates. At age 8, effects on the Wechsler Intelligence Scale for Children Full and Verbal scales for children who attended > 400 days ranged from 7 to 10 points. For the heavier LBW infants (2,001-2,500 g), the effects were about 14 points for > 400 days; for the lighter LBW infants (< or = 2,000 g), the effects were about 8 points. These effects were all substantially higher than corresponding ITT effects. Similar but smaller effects were found for children who attended > 350 days.