Three children presented with a complex syndrome of atypical psychotic and extremely immature behavior, obesity and overgrowth, borderline retardation, and seizures (prominent in two). Weight overgrowth exceeded height overgrowth and was stratospheric (up to 8 SD above mean). Obesity seemed related to lack of satiety. The cases fit no known condition: hypothalamic damage, Sotos' syndrome, and Prader-Willi syndrome were excluded. Empirical treatment with anticonvulsants (carbamazepine and acetazolamide) together with psychotropic agents (selective serotonin reuptake inhibitors and risperidone) controlled seizures, improved behavior, and stopped weight gain in each patient. We have not found this syndrome previously described. The etiology is unknown: perinatal encephalopathy could be a factor in the two patients with prominent seizures; in the third, familial major affective disorder is implicated. Medication responses suggest a low-serotonin state underlying the lack of satiety, an imbalance of serotonin and noradrenergic modulation in the hypothalamus, and epileptogenic disorders (or affective disorder responsive to anticonvulsants in one case) involving these same systems.