Sex differences in children's play patterns during middle childhood are thought to promote greater awareness of social acceptance among girls compared with boys. The present study posited that girls are more discerning of peer acceptance than are boys; however, these sex differences were predicted to vary depending on how discrepant perceptions were assessed (i.e., inaccuracy versus bias). Additional differences were expected if children perceived acceptance by same- versus opposite-sex peers. Participants were 912 third through fifth graders (420 girls and 492 boys). Consistent with predictions, boys were more inaccurate than girls, but only for perceived acceptance by same-sex peers. As expected, girls were more negatively biased than boys, but only for perceived acceptance by opposite-sex peers. Results did not support the hypothesis that boys have more positively biased perceptions of peer acceptance than girls. Overall, these findings raise important issues regarding the evaluation of children's discrepant self-perceptions of peer acceptance. (Contains 1 table and 2 figures.)