This study examined the similarity among 45 sibling pairs in the third through fifth grade, in their social and academic adaptation to the school setting as evaluated by multi-agent and multi-method assessment of adjustment. Measurements included teacher ratings and rankings of academic skills, social behavior, and peer acceptance; peer sociometric ratings by all classmates; and direct observations of children on the playground with peers and in the classroom with teachers and peers. Comparisons were made with a set of randomly selected unrelated subject pairs matched with each sibling on sex, grade, and classroom. Significant correlations were found only among sibling pairs on peer ratings of social preference, teachers' judgments of academic competence (math and reading), popularity, social behavior and school adjustment, positive behavior with peers on the playground, and teachers' disapproving behavior in the classroom. Results provided support for a social interactional perspective of shared environmental influences on sibling similarity, and underscored the need for more multi-agent and method research on sibling concordant and discordant adjustment. (Author/MM)