A 47 year-old woman with juvenile myoclonic epilepsy received anticonvulsant therapy with valproate, 200 mg 4 times a day. Severe abdominal pain and vomiting developed 3 days after initiation of therapy, and she became comatose requiring artificial ventilation. Increased urinary output of delta-amino-levulinic acid (48 uM/24 hr, 8 times normal) and porphobilinogen (9 uM/24 hr, twice normal) were demonstrated. Complete recovery took place 5 days after discontinuation of valproate.