OBJECTIVE: The authors’ goal was to determine the frequency of psychiatric comorbidity among treatment-seeking pathological gamblers, compare the severity of gambling and psychological problems in gamblers with and without comorbid disorders, and investigate differences between gamblers with and without comorbid disorders in the dopamine D2 receptor gene (DRD2). METHOD: Sixty-nine pathological gamblers who consecutively applied to a specialized outpatient treatment program were evaluated with structured interviews, self-report questionnaires, and psychological scales and were genotyped for a DRD2 polymorphism. RESULTS: A comorbid psychiatric disorder was present in 43 (62.3%) of the gamblers. The most frequent diagnoses were personality disorders (N=29 [42.0%]), alcohol abuse or dependence (N=23 [33.3%]), and adjustment disorders (N=12 [17.4%]). Gamblers with comorbid psychiatric disorders had gambling scores and psychological scale scores indicating greater severity of gambling and psychopathology. Significant differences in DRD2 allele distribution were found in gamblers with and without comorbid disorders. CONCLUSIONS: Psychiatric comorbidity is common among pathological gamblers and is associated with greater severity of clinical problems. The DRD2 gene could be a liability genetic factor for psychiatric comorbidity in pathological gambling.