© 2015 American Medical Association. All rights reserved. Importance: Tourette syndrome (TS) is characterized by high rates of psychiatric comorbidity; however, fewstudies have fully characterized these comorbidities. Furthermore, most studies have included relatively fewparticipants (< 200), and none has examined the ages of highest risk for each TS-associated comorbidity or their etiologic relationship to TS.Objective: To characterize the lifetime prevalence, clinical associations, ages of highest risk, and etiology of psychiatric comorbidity among individuals with TS.Design, Setting, And Participants: Cross-sectional structured diagnostic interviews conducted between April 1, 1992, and December 31, 2008, of participants with TS (n = 1374) and TS-unaffected family members (n = 1142).Main Outcomes And Measures: Lifetime prevalence of comorbid DSM-IV-TR disorders, their heritabilities, ages of maximal risk, and associations with symptom severity, age at onset, and parental psychiatric history.Results: The lifetime prevalence of any psychiatric comorbidity among individuals with TS was 85.7%; 57.7%of the population had 2 or more psychiatric disorders. The mean (SD) number of lifetime comorbid diagnoses was 2.1 (1.6); the mean number was 0.9 (1.3) when obsessive-compulsive disorder (OCD) and attention-deficit/hyperactivity disorder (ADHD) were excluded, and 72.1% of the individuals met the criteria for OCD or ADHD. Other disorders, including mood, anxiety, and disruptive behavior, each occurred in approximately 30% of the participants. The age of greatest risk for the onset of most comorbid psychiatric disorders was between 4 and 10 years, with the exception of eating and substance use disorders, which began in adolescence (interquartile range, 15-19 years for both). Tourette syndrome was associated with increased risk of anxiety (odds ratio [OR], 1.4; 95%CI, 1.0-1.9; P = .04) and decreased risk of substance use disorders (OR, 0.6; 95%CI, 0.3-0.9; P = .02) independent from comorbid OCD and ADHD; however, high rates of mood disorders among participants with TS (29.8%)may be accounted for by comorbid OCD (OR, 3.7; 95%CI, 2.9-4.8; P < .001). Parental history of ADHD was associated with a higher burden of non-OCD, non-ADHD comorbid psychiatric disorders (OR, 1.86; 95%CI, 1.32-2.61; P < .001). Genetic correlations between TS and mood (RhoG, 0.47), anxiety (RhoG, 0.35), and disruptive behavior disorders (RhoG, 0.48), may be accounted for by ADHD and, for mood disorders, by OCD.Conclusions And Relevance: This study is, to our knowledge, the most comprehensive of its kind. It confirms the belief that psychiatric comorbidities are common among individuals with TS, demonstrates that most comorbidities begin early in life, and indicates that certain comorbiditiesmay be mediated by the presence of comorbid OCD or ADHD. In addition, genetic analyses suggest that some comorbiditiesmay be more biologically related to OCD and/or ADHD rather than to TS.