The purpose of the present study was to examine the impact of family relationships and internalizing problems on psychosocial adjustment. Data were collected from 404 Polish children and early adolescents aged 8-13 using standardized instruments to assess the quality of family relationships (i.e., control, support), internalizing problems, and psychosocial adjustment (problem behavior versus prosocial behavior). The findings confirmed positive and negative associations between the quality of family relationships, internalizing problems, problem behavior, and prosocial behavior. Gender differences were also found; girls received more support within family relationships and scored higher in prosocial behavior, whereas boys received more control within family relationships. Regression analyses have shown that control within family relationships and internalizing problems were positive predictors of problem behavior. On the other hand, control within the family relationships was a negative predictor of prosocial behavior. In the final step, four mediation models were tested to check whether internalizing problems would mediate the relationship between family relationships and the child's behavior. Possible future research directions are discussed.