Objective: To investigate the prevalence and type of comorbidity in children with gender identity disorder (GID). Method: The Diagnostic Interview Schedule for Children--Parent Version was used to assess psychopathology according to the DSM in two groups of children. The first group consisted of 120 Dutch children (age range 4-11 years) who were referred to a gender identity clinic between 1998 and 2004 (GID group) and the second group consisted of 47 Dutch children who were referred to an attention-deficit/hyperactivity disorder (ADHD) clinic between 1998 and 2004 (ADHD group; 100% response rate for both groups). Results: Fifty-two percent of the children diagnosed with GID had one or more diagnoses other than GID. As expected, more internalizing (37%) than externalizing (23%) psychopathology was present in both boys and girls. Furthermore, the odds ratios of having internalizing or externalizing comorbidity were 1.28 and 1.39 times higher, respectively, in the clinical comparison group (ADHD group) than in the GID group. Finally, 31% of the children with GID suffered from an anxiety disorder. Conclusions: The results of this categorical diagnostic study show that children with GID are at risk for developing co-occurring problems. Because 69% of the children do not have an anxiety disorder, a full-blown anxiety disorder does not seem to be a necessary condition for the development of GID. Clinicians working with children with GID should be aware of the risk for co-occurring psychiatric problems and must realize that externalizing comorbidity, if present, can make a child with GID more vulnerable to social ostracism. (Contains 3 tables.)