The aim of present study was to examine the relationship between young children's peer-reported trustworthiness and their school adjustment. Two hundred and eleven (103 male and 108 female) children in the United Kingdom (mean age = 6 years 2 months) took part in this study. Measures of peer-reported trustworthiness, child-rated school adjustment, and teacher-rated school adjustment were administered twice across a one-year period. Also, children's number of friendships, peer acceptance, and self-control were assessed at Time 2. Multisample path analyses were conducted separately by sex. For both samples there were direct longitudinal paths between peer-reported trustworthiness and changes in teacher-rated school adjustment. For boys, the longitudinal path between peer-reported trustworthiness and changes in child-rated loneliness was mediated by peer acceptance, and peer-reported trustworthiness mediated the relationship between self-control and teacher-rated school adjustment. Sex differences in peer-reported trustworthiness also emerged: girls were rated as more trustworthy by their peers than were boys. The findings support the hypothesis that young children's trustworthiness contributes to school adjustment, which is due in part to peer acceptance. (Contains 6 figures, 2 tables and 2 notes.)