Tourette syndrome (TS) is a childhood onset neurodevelopmental disorder characterised by motor and vocal tics and frequent associated comorbidities. The developmental trajectory of tic shows tic-onset in the age of 4-6, peak in the age of 10-12 and decline during adolescence, although only few and small longitudinal studies form the basis of this evidence. Recent studies suggest that comorbid obsessive-compulsive disorder (OCD), attention deficit-hyperactivity disorder (ADHD) and coexisting psychopathologies tend to persist and become more dominant in adolescence. This large prospective follow-up study want to examine the clinical course of TS: tic and comorbidities during adolescence, the prevalence of coexisting psychopathologies, the tic-related impairment, development in phenotype expression and find predictors for the expected course of TS. Method: This study is examining a large clinical cohort recruited at the Danish National Tourette Clinic during the period 2005-2007 and 2011-2013. At baseline, 314 participants aged 5-19 years were included and at follow-up 6 years later 227 participated, aged 11-26. All participants were uniformly clinically examined at basis and follow-up with a clinical interview and validated measurements to assess comorbidities. The Yale Global Tic Severity Scale was used to asses tic severity and tic-related impairment. At follow-up a cross-sectional diagnostic evaluation was made with the Development and Well-Being Assessment to assess coexisting psychopathologies. Results: A significant decline in tic and the most frequent comorbidities OCD and ADHD was found although some variation existed and some subclinical and partial remissions persisted. Tic-related impairment was not reflected in the tic-decline as expected but influenced by several parameters. The phenotype expression was found to be dynamic but overall changed toward TS without comorbidities. Several predictors were found to predict the clinical course of TS in adolescence and early adulthood. Childhood tics, OCD and ADHD severity were the strongest predictors for future symptoms of the respectively diagnoses. Comorbidities and coexisting psychopathologies were found in 63% at follow-up, whereas 37% had pure TS. Conclusion: The clinical course of TS during adolescence was confirmed, with solid evidence, with decline in tics, OCD and ADHD severity. We provide evidence of considerable coexisting psychopathologies requiring clinical support and partial remissions and subthreshold symptoms requiring monitoring and clinical guidance to assist the young adults in promoting a healthy transition into early adulthood. Furthermore we provide predictors for the clinical course of TS to be used in the preventive efforts, early intervention and allocation of resources improving quality of life for the children and their families.