Four patients with glycogen storage diseases have been treated with diphenylhydantoin for more than two years. A good response, characterized by a reduction in liver size both clinically and by liver scan and a reduction in hepatic glycogen content by chemical analysis and light and electron microscopy, was observed. Hyperlacticacidemia improved. Some changes in blood glucose and liver function tests also were observed. Although the most likely mechanism of action is the induction of the deficient hepatic enzymes, we were not able to demonstrate this. Diphenylhydantoin appears to be effective in the treatment of types I, III, and VI of glycogen storage diseases. Clinical trials in a larger group of patients including those with the more serious types of the disease is warranted before the efficacy of the drug is confirmed.