Since it is clear from the literature and from this research that both problematic mental states and behaviors are linked, research should examine both factors when examining developmental patterns during adolescence. Research should also address the determinants of change in these factors for both boys and girls. Problematic mental states (anxiety/depression, withdrawn, and physical complaints) and behaviors (delinquency, aggression, school problems, and disobedient to parents) were found to be distinctive features of early adolescence but were linked in their occurrence, i.e., when problem behaviors occurred, problematic mental states were also present. This circumstance remained stable over time in the course of adolescence for both boys and girls; however, more girls than boys scored high on problematic mental states, as expected. The study sample was drawn from three secondary schools in the Netherlands. At time one, the sample consisted of 650 eighth-grade adolescents between 12 and 15 years old. A battery of questionnaires was administered to the sample to measure problematic mental states and behaviors. After a 1-year interval, the same adolescents were tested again, yielding data from 563 members of the original sample. 4 tables, 2 figures, and 55 references