Cyclic vomiting syndrome (CVS) is a disorder characterized by recurrent, stereotypic episodes of incapacitating nausea, vomiting, and other symptoms, separated by intervals of comparative wellness. Associated symptoms include nausea, abdominal pain, headache, and motion sickness. Recently, CVS was categorized as a migraine. Case 1 was a girl aged 4 years and 11 months, who had frequent and severe episodes of vomiting since she was 3 years old. The diagnosis of CVS was established on the basis of clinical symptoms and laboratory data. Her electroencephalogram was normal. Prophylactic therapy using a single drug such as amitriptyline, carbamazepine, phenytoin, cyproheptadine, valproate sodium or phenobarbital was not effective. However, her recurring vomiting disappeared with prophylactic therapy using valproate sodium and phenobarbital. Case 2 was a boy aged 10 years and 7 months, who had frequent episodes of vomiting since he was 1 year and 10 months old. He had been receiving intravenous hyperalimentation therapy at home since infancy because of frequent vomiting and failure to thrive. His electroencephalogram showed no abnormality. Prophylactic therapy using a single drug such as diazepam, phenytoin, valproate sodium or phenobarbital was not effective. However, his recurring vomiting disappeared with prophylactic therapy using valproate sodium and phenobarbital. There were no adverse effects in both patients. The combination therapy with valproate sodium (20 - 26 mg/kg/day) and phenobarbital (4 - 5 mg/kg/day) was effective as a prophylactic therapy in these two patients. The combination therapy with valproate sodium and phanobarbital for prophylaxis of vomiting may be helpful in patients with intractable CVS.