The authors administered measures of loneliness, generalized trust beliefs in peers, and trust beliefs in specific familiar peers (i.e., opposite-gender peers, same-gender peers, close same-gender peers) to a sample of 63 children (33 girls, 30 boys) from 4th and 5th grades (M age = 10 years, 6 months). They assessed children's trusting behavior by engaging them in a Prisoner's Dilemma game (reciprocal trusting) and by evaluating teachers' ratings. The authors found that, across gender, loneliness was negatively correlated with each measure of trust beliefs and trusting behavior. As expected, the relationship between children's loneliness and trust, specifically trust beliefs in same-gender peers, was stronger for girls than for boys. In support of an additive risk model, the authors found low trust beliefs in same-gender peers and low reciprocal trusting behavior with peers each statistically contributed to loneliness in girls. (Contains 3 tables.)