Background: Mental health problems are common in adolescence and seeking help for them is becoming more common. Referrals to adolescent mental healthcare have recently increased in Finland. Objective: To examine time trends in internalizing and externalizing mental health symptoms among Finnish adolescents. Method: A time-trend school survey was conducted among 9 th graders (15-year-olds) in Tampere, Finland, in three time periods : 2002-03, 2012-13 and 2018-19 (N = 4,162). Results: Compared to the period 2002-03, prevalence of externalizing symptoms decreased in the period 2012-13 and further in 2018-19. The prevalence of internalizing symptoms did not change significantly between 2002-03 and 2012-13; however, in 2018-19, depression, social anxiety, general anxiety, poor subjective health, stress symptoms among boys, and poor selfesteem increased compared to earlier time periods. The increases were more marked among girls. However, suicidal ideation did not increase in 2018-19 compared to earlier time periods.
Conclusion:Whereas the prevalence of externalizing symptoms decreased among Finnish adolescents between 2002-03 and 2018-19, the prevalence of internalizing symptoms increased between 2012-13 and 2018-19. To help to understand the causes of these increases and to prevent internalizing problems, further research on the underlying causes is needed.