A 3-month-old male and a 4-month-old female infant with intractable seizures were diagnosed as having malignant migrating partial seizures in infancy (MMPSI) with developmental arrest on the basis of characteristics of symptoms, clinical courses and EEGs. We treated these two patients with potassium bromide (80 mg/kg) after conventional antiepileptic drugs failed to adequately control the seizures. The potassium bromide therapy resulted in complete control of seizures in one patient, and more than 95% reduction in seizure frequency in the other.