This study examined the effects of the Child Development Project, an elementary school program designed to enhance prosocial development on children's peer relations and social adjustment. Research has focused on a longitudinal cohort of children that entered three program schools in kindergarten in the fall of 1982, and completed sixth grade in the spring of 1989. A corresponding cohort of children that attended three other elementary schools in the same district has served as a comparison group. Data were collected from cohort students at all six schools at third and fourth grades, from students at two schools at fifth grade, and from students at four schools at sixth grade. Overall, 236 comparison and 285 program students participated in the research. Data revealed that program students were more accepting of (and more accepted by) their classmates, were less lonely, and were lower in social anxiety than were students at comparison schools. No differences were found between program and comparison students on measures of self-esteem, liking for school, perceived social competence, or perceived popularity. The observed positive effects on peer relations and social adjustment support and extend earlier findings indicating that the program has had positive effects on students' interpersonal behavior in the classroom, social problem solving skills, and commitment to democratic values. Collectively, these studies suggest that the program has consistent positive effects on children's social development throughout the elementary school years. (Author/NB)