This research addressed the issue of developmental changes in intimacy among same-sex and cross-sex adolescent friendship dyads. Drawing on the theories of Sullivan (1953) and Selman (1971; 1980), the study was designed to compare a same-sex friend with a cross-sex friend on knowledge of and sensitivity to a third (target) person on five subscales: Academic Attitudes, Social Preferences, Individual Activities, Values, and Future Orientation. Subjects were 72 seventh graders, 60 ninth graders, and 66 eleventh graders. A 3 (grade) x 2 (sex of target) x 2 (sex of friend) multivariate analysis was performed, using a split-plot factorial design. Correlational analyses were also performed. Results indicate a developmental trend in intimacy, with older subjects having more accurate perceptions of their friends, but expected sex differences did not emerge consistently. Descriptive analyses of age and sex differences on the five subscales using a larger sample were also carried out. Discussion and interpretation of these results are included.