A randomized controlled trial involving 128 infants born prematurely compared basic developmental care (nests and incubator covers) and the Newborn Individualized Developmental Care and Assessment Program (NIDCAP) intervention (behavior observations and guidance by a trained developmental specialist) in relation to effects on parental stress and infant behavior and temperament during the first year of life. Children in the NIDCAP group who received intervention for more than 1.5 months demonstrated more social-relatedness behavior at 1 year. Parents in the NIDCAP group reported that the intervention provided positive experiences for them and reported a positive effect on the well-being of their infant. No statistically significant or noteworthy effects were found between the groups on measures of temperament, problem behavior, or parental stress. Implications for NIDCAP implementation are discussed.