Background: Social integration of children and quality of peer relationships are manifestations of social competence and related to the development of behavior disorders. Objectives: Can strengths and difficulties of children be assessed with good reliability? Which relationship exists between sociometric indices of children? Methods: We studied 642 students aged nine to fourteen using the SDQ-L and two sociometric questions (choice of classmates to sit next to). Results: The psychometric quality and the factorial validity of the SDQ-L can be judged as good. Data of teachers about problems with peers and about externalising behavior disorders were confirmed by the passive choices of students (less positive and more negative passive choices) and depend on the reciprocity of students (less engagement and more rejection). Conclusions: All three sources of data (ratings of teachers and classmates, self-descriptions of students) are mutually connected. Thus, the SDQ-L can give indications about problematic peer relationships and offer possibilities for preventive interventions.