Peer nominations and teacher ratings were used to examine age and sex differences in behaviors associated with perceived coolness during middle childhood. Participants were 470 students in Grades 1, 3, and 5. Participants nominated peers whom they perceived as cool; separate scores were calculated for samesex coolness and cross-sex coolness (i.e., cool nominations received from samesex and cross-sex classmates, respectively). Teachers reported on children's prosocial and aggressive behavior. Behavior--coolness associations differed by (a) sex of the target child, (b) age of the target child, and (c) sex of the perceiving child. Most notably, aggressive behavior positively predicted cross-sex coolness for girls (not boys) and for students in Grades 3 and 5 (not Grade 1). Results underscore the value of disaggregating peer nominations by sex. Discussion builds upon the literature on perceived coolness and popularity, which has drawn disproportionately from adolescent samples.