Aim. To determine the extent to which mental disorders (depressive disorders, anxiety disorders, eating disorders) and psychoactive substance use are prevalent among Krakow secondary school adolescents in the late adolescent phase. Method. A representative sample of the population of Krakow secondary school pupils was tested. A two-stage draw method identified a group of 1933 2nd form pupils of all types of secondary schools: grammar schools, technical schools and vocational schools (17-year olds). They were tested using the following screening questionnaires: Beck depression scale, EAT-26 eating disorders scale, Obsessive-compulsive disorder (Leyton) scale and author's drug questionnaire. Results. Girls declared the occurrence of symptoms of mental disorders twice as often than boys (41.2% vs. 22.7%). Among girls, the most common disorders were: depressive symptoms (33.6%), eating disorder symptoms (18.7%) and obsessive-compulsive disorder symptoms (5.4%). And in the boys group: depressive symptoms (22.7%), eating disorder (7.3%) and obsessive- compulsive symptoms (3.4%). Findings were obtained concerning the prevalence of substance use in the boys and girls group (73% vs. 68%), mainly alcohol, as well as alcohol and cigarettes and alcohol, cigarettes and drugs. Conclusions. The prevalence of mental disorders in the population of 17-year-olds is significant, it concerns especially depressive symptoms and eating disorder symptoms. A significant prevalence of psychoactive substance use was found in this age group, mainly alcohol and mixed type substance use: alcohol and cigarettes and alcohol, cigarettes and drugs.