Transactional relations between children's positive social interaction skills, school engagement, and academic achievement were examined using a longitudinal panel model across the transition from preschool to first grade. Participants were Head Start children (N = 241; 49% girls, [mean] age = 53 months, range 45-60); 78% were Mexican/Mexican-American; 82% of families were of low socioeconomic status. Head Start children's positive social interaction skills and academic achievement in preschool were positively related to kindergarten school engagement, positive social interaction skills and school engagement influenced one another over time, and academic achievement was positively related to positive social interaction skills from preschool to kindergarten. A small, but significant, transactional effect of preschool academic achievement on first-grade school engagement through kindergarten positive social interaction skills was found. Findings from the current study provide support for previously undocumented longitudinal relations between positive social interaction skills, school engagement, and academic achievement for Head Start children. (author abstract)