Purpose <p>Attention Deficit Hyperactivity Disorder (ADHD) and concurrent psychiatric comorbidities have been thoroughly investigated but less is known on how ADHD symptoms predict subsequent psychiatric disorders. The aim of this study is to examine whether ADHD symptoms in adolescence predict adult psychiatric disorders in a longitudinal unselected population-based cohort.</p> Methods <p>The study is based on the Northern Finland Birth Cohort 1986 (<i>N</i> = 9432), in which ADHD symptoms had been measured with the parent-filled Strengths and Weaknesses of ADHD Symptoms and Normal Behaviours (SWAN) questionnaire at the age of 16 years. Adult psychiatric disorders were retrieved from the nationwide registers up to age 35 years. The population was divided into tertiles based on ADHD symptoms and the correlations between middle and highest tertiles and certain later psychiatric disorders were examined with logistic regression analyses, taking family type, parents’ education and adolescent-reported psychiatric symptoms as covariates.</p> Results <p>Among females, ADHD symptoms in adolescence predicted later anxiety and depressive disorders in both the crude and adjusted models for the middle and highest tertiles and substance use disorders in the highest tertile. The highest risk was associated with substance use disorders. In males, only anxiety disorders could be predicted for the middle and highest tertiles in all the analyses.</p> Conclusions <p>ADHD symptoms in adolescence predict multiple psychiatric disorders among females and anxiety disorders among males. ADHD symptoms should be addressed as soon as they appear even if they do not fulfil the diagnostic criteria for ADHD. More research needs to be done into this question.</p>