Background: Research points in different directions when looking at possible increases in mental health problems among adolescents. Findings in favor of an increase are questioned due to methodological problems. Aim: Investigating whether mental health problems among young adolescents are increasing over time in Europe and North America. If so, does the trend apply both to mean levels of symptoms and to the proportion of adolescents with substantial problems? Are the time-trends similar over sex and age-categories?                                                                                                    Method: A total of 401 089 adolescents from a total of 38 countries are included in the analysis. Based on the eight health variables on self-rated health provided by the HBSC study, a measurement of mental health problems was created using factor analysis in SPSS. Results: Increases of mental health problems were found in Europe and North America. Increases were found both in terms of mean levels of symptoms and to the proportion of adolescents with substantial problems. Increases were seen in all age groups and among both girls and boys. Conclusion: Reasons behind the discovered increases are not known and should be further investigated as extensive research point to severe consequences of mental health problems in adolescence for later life.       Key words   Adolescents, mental health problems, trends, self-reported health (SRH), psychological health complaints (PHC)