1 or 2 concurrent maintenance antiepileptics required At least 6 months since corticotropin At least 60 days since acetazolamide or zonisamide At least 60 days since investigational drug or device No ketogenic diet --Patient Characteristics-- Hematopoietic: No hematological abnormality within 2 years Hepatic: No hepatic disease within 2 years Renal: No nephrolithiasis No other renal disease within 2 years Cardiovascular: No clinically significant electrocardiographic abnormality No cardiovascular disease within 2 years Pulmonary: No respiratory disease within 2 years Other: Weight at least 25 lb (11.5 kg) No medical disease within 2 years, e.g.: Rheumatic fever Gastrointestinal abnormality Malignancy No psychiatric or mood disorder inconsistent with Lennox-Gastaut within 6 months requiring any of the following: Electroconvulsive therapy Antidepressants Anxiolytics Antipsychotics Lithium carbonate No history of alcohol or drug abuse No history of poor compliance on past antiepileptic therapy Able to take medication and maintain seizure calendar (assistance allowed) Adequate parental supervision 1 parent/guardian with adequate English fluency if English not patient's primary language The following required of fertile women: Negative serum-beta pregnancy test immediately prior to entry Normal menstrual flow for 3 months prior to entry Medically acceptable form of contraception during study