Introduction: Glycogen storage diseases (GSD) are a group of autosomally recessiveinherited disorders due to defects in the metabolism of glycogen. The liver and skeletalmuscles tend to be the most severely affected organs. The most common manifestations arehypoglycaemia, convulsions, hepatomegaly, growth retardation and myopathy.The aim of this work: was differentiate between type І and type Ш GSD by clinicaland biochemical characters. The second aim was to determine the fasting tolerance of eachpatient to prevent attacks of hypoglycaemia and acidosis and to improve growth.Patients and methods: The study included 41 patients with GSD presented to thePediatric Hepatology Unit, Cairo University Children’s Hospital, in the time period fromAugust, 2009 to February, 2011. Investigations done included complete blood count, liverfunctions, prothrombin time, serum lactate, triglycerides, uric acid, creatine kinase (CK)and enzyme assay for GSD. Fasting tolerance test was done for all patients, followed byglucagon test for the patients who developed hypoglycaemia.Results: The patients’ ages ranged between 6 months to 11.5 years, mean 4.4 + 2.9years, median: 3.5 years (IQR: 3.2). Twenty five patients were males (61%). GSD III waspresent in 31/41 patients. Our results showed that 96.8% of patients with GSD type Ш havehypertriglyceridemia, 51.6% have hyperlactic acidemia and 20% have hyperuricemia.Nineteen patients (61%) of GSD type III had elevated CK. There was significantcorrelation between the age of the patients and the level of CK (r=0.7 & P=0.000). Fastingtolerance test was done for all patients. Thirty patients (73.1%) developed hypoglycaemiaand glucagon test was done. Only 6 patients (14.6%) responded to glucagon administration.According to the fasting tolerance, we recommended for each child his/her new regime forstarch administration. Six months later, we compared patients’ growth velocities before andafter intervention. The results showed significant improvement of the growth velocity(p=0.000) and reduction in the frequency of seizure attacks (p=0.0005) and size of the liver(p=0.02). Serum lactate improved in only 8 (19.5%) of the patients.Conclusion: Adjusting the intervals between the cornstarch doses for each patient withGSD, according to individual fasting tolerance test was very beneficial. This resulted inimprovement of the growth velocity and reduction in the frequency of hypoglycaemicseizures as well as liver size.