OBJECTIVE: To compare rates of DSM-IV psychiatric disorders between (1) a clinical study group of extremely obese adolescents and young adults, (2) gender-matched population-based obese controls and (3) a population-based control group of the same age range. DESIGN: Rates of psychiatric disorders were assessed in (1) the clinical study group of obese adolescents and (2) the population based sample of obese adolescents, and compared to (3) a large population-based control group using a standardized psychiatric interview. SUBJECTS: (1) Clinical study group: 30 female and 17 male extremely obese adolescents and young adults (age range: 15-21 y; mean BMI:42.4 kg/m2). (2) Thirty females and 17 males with the highest BMI (age range 15-21 y; mean BMI:29.8 kg/m2) of a population-based control group encompassing 1655 (805 males) adolescents and young adults. (3) The population based control group excluding the 30 females and 17 males with the highest BMI (n=1608; 788 males). MEASUREMENTS: Munich-Composite International Diagnostic Interview (M-CIDI) allowing for DSM-IV diagnoses. RESULTS: High rates of mood, anxiety, somatoform and eating disorders were detected in the clinical sample of obese adolescents which exceeded those observed in population controls (all P-values <0.01). Rates between population-based obese adolescents and young adults and population controls did not differ. In most patients the psychiatric disorders set in after onset of obesity. 57% and 35% of the female and male patients, respectively, reported eating binges with lack of control. However, less than one-half of these patients qualified for a DSM-IV diagnosis of an eating disorder. CONCLUSIONS: Extremely obese adolescents and young adults who seek long-term inpatient treatment have a high lifetime prevalence for affective, anxiety, somatoform and eating disorders. Because the mean BMI of the clinical study group was considerably higher than that of the obese population controls, we were not able to clarify whether the high rate of psychopathology in the study group was related to the extreme obesity or to their treatment-seeking behavior.