Tourette syndrome (TS) is a common, neuropsychiatric disorder which has many similarities to attention deficit hyperactivity disorder (ADHD). TS probands have a high frequency of a variety of behavioral disorders including depression. The depression may be due to a pleiotrophic effect of the Gts genes, proband ascertainment bias, or a result of coping with the chronic tics. To distinguish between these hypotheses we examined the responses to 17 Diagnostic Interview Schedule questions to evaluate the 9 DSM-III-R criteria for major depressive episode in 1,080 adults consisting of TS and ADHD probands, their relatives and controls. Using a Bonferonni corrected p there was a significant progressive increase in 16 of 17 depressive symptoms and for a life time history of a major depressive episode in groups with increased genetic loading for Gts genes. Similar trends were seen in the small number of ADHD probands and their relatives. There was also a significant increase for these variables in non-proband TS relatives versus non-TS relatives, indicating the association of depression with Gts genes was not due to ascertainment bias or the inappropriate choice of controls. Multiple linear regression analysis indicated that obsessive-compulsive behaviors, sex, ADHD, drug abuse, and age all showed a more significant effect on depressive symptoms than the number of tics. The presence or absence of TS in the relatives had a much greater effect on risk for depression than the presence or absence of an episode of major depression in the proband. These results are consistent with the hypothesis that Gts and ADHD genes play a major role in depression.