Introduction: Mental disorders begin at an early age. Screening is the first step in early intervention for the mental health problems of children and adolescents. The aim of this study was to analyse the prevalence of mental health problems among adolescents from Osijek, to early detect the risks of possible mental disorders, and to analyse gender differences. Materials and methods: The main screening instrument was The Strengths and Difficulties Questionnaire (SDQ), which assesses the emotional and behavioural difficulties of children and adolescents, as well as the prosocial skills. Over 5 school years (2012/13-2016/17), a total of 5787 students were examined. 5514 (95.3%) first-year students from 16 high schools and 273 (4.7%) seventh-grade students from 10 elementary schools were examined in the City of Osijek. Results: 10.4% of students had the borderline or abnormal score on the Total difficulties score. Girls had higher scores on the Total difficulties score (p<0.01), Emotional symptoms subscale (p<0.01), Prosocial behaviour subscale (p<0.01), and on the Hyperactivity/inattention subscale (p<0.01). On the Conduct problems subscale and the Peer relationships problem subscale, boys scored significantly higher than the girls (p<0.01). Conclusion: In the Osijek area, girls are more affected by mental problems than boys, scoring higher on the total scale, as well as on the emotional and hyperactivity scale. Whereas boys scored higher on the conduct and peer relationship scales. The data obtained through this screening allows for further planning of public health measures, preventive activities, treatment, and mental health promotion.