Numerous studies indicate age and sex differences in adolescent interests and self-concept development; however, few longitudinal studies have been conducted outside the area of vocational interests. It was hypothesized that an examination of data from both cross-sectional and longitudinal samples would clarify age and sex differences in the development of adolescent self-concept. Male and female elementary, middle, and high school students completed questionnaires, a self-concept scale, interest assessments, and instruments about the sources of information used by adolescents. Results indicated that cognitive functioning was a primary determinant of self-concept. Environmental encounters such as peer/family relationships, school, role-taking, and reactions of others affected self-concept. The longitudinal data demonstrated consistency in self-concept measures over time. Cross-sectional comparison data, showing grade-level differences, did not replicate each other. Interest in interpersonal relationships, education, human ecology, and drugs was higher for older students than younger students. Social class and academic achievement had little effect on self-concept development. (Author/HLM)