Juvenile delinquency is a negative social phenomenon closely linked to the environmental factors that affect the lives of individuals. For adolescents, the family and school environments are important in this context. The study focuses on an analysis of the impact of parenting (parental warmth, parental control, parental involvement and inconsistent parenting) and factors in the school environment (positive attachment to school, negative school climate, feeling of safety, teacher support, and academic motivation) on the occurrence of delinquency with regard to its severity (behavioural problems, less severe delinquency, severe delinquency) and the sex of the adolescent. The results of a regression analysis show that family and school environmental factors combine in the aetiology of delinquency. Generally, the most influential protective factors were academic motivation and parental warmth, while the most influential risk factors were inconsistent parenting and a negative school climate. While less severe delinquency and behavioural problems in males seemed to be more sensitive to factors in the school environment, the same types of female delinquency were more sensitive to parental warmth. In cases of severe delinquency, the situation is turned around – the most protective factor in males was parental warmth and the highest risk factor in females was negative school climate.