This paper investigates the formation and evolution of friendship cliques among preadolescent youths. An understanding of these patterns can aid teachers in influencing social relations and the behavior of their students. Two sets of sociometric data were collected to analyze children's friendship cliques. The relationship among the variables of class size, classroom organization, grade, and sex was examined in a cross-sectional data set of 51 groups and a longitudinal data set of 11 groups. Class size and organization were found to have an effect on average number of friends and on the number, size, and stability of cliques. Fewest children excluded from cliques were found in the sixth grade. While some cross sex friendships were reported, a total sex cleavage existed between boys' and girls' cliques in every class. Data, research methodology, findings, and statistical tables are included in the report. (Author/KC)