Trustworthiness was examined in children and early adolescents from two countries. In Study 1,505 children in the fifth and sixth school years in the United Kingdom (mean age = 9 years 7 months) were tested across an 8-month period. In Study 2,350 sixth- through eighth-grade Canadian children and early adolescents (mean age = 12 years 11 months) were tested across a 1-year period. Participants completed measures assessing trustworthiness (ratings of promise and secret keeping by peer group and also by best friend in Study 1), assertiveness (Study 1), prosocial behavior (Study 2), social relationships (friendships and peer preference), and psychological adjustment (self-esteem, depressive symptoms, and loneliness). Peer-reported trustworthiness was positively associated with, and predicted changes across time in, the number of friendships. The observed relations were found to be statistically independent of peer preference and peer-reported assertiveness or prosocial behavior. Trustworthiness was found to be positively associated with peer preference (Study 1 and Study 2) and psychological adjustment (Study 1).