Individuals with type Ia glycogen storage disease (glucose-6-phosphatase deficiency) frequently develop hepatic adenomas. Potential complications involving these adenomas include malignant transformation and hemorrhage. Five of 9 patients with this disease had evidence of hepatic filling defects on radionucleotide liver scan when first evaluated at our hospital. Dietary therapy aimed at preventing hypoglycemia was begun in 7 of the 9 patients. Prevention of hypoglycemia resulted in the correction of all of the metabolic abnormalities (lactic acidosis, hyperlipidemia, hyperuricemia, and growth retardation). Treatment also corrected the marked elevation in plasma glucagon concentrations. A disappearance of the hepatic lesions occurred in 2 of the treated patients, and a marked reduction in size of the adenoma occurred in the third patient. The hepatic filling defects remained present in the two untreated patients. None of the affected patients receiving dietary therapy have developed hepatic adenomas. One of these patients is now 22 yr old and has received dietary therapy for 7 yr. Early dietary therapy seems to be effective in preventing development of adenomas as well as inducing their resolution.