Three children from two German families are described and the observations compared with the previously published three families comprising eight patients. The two index cases presented with morning fatigue, had ketotic hypoglycaemia when fasting which rapidly disappeared after eating, and hepatic glycogen deficiency and absent or very low hepatic glycogen synthase activity. Metabolic profiles comprising glucose, lactate, alanine, and ketones in blood were typical for hepatic glycogen synthase deficiency. Symptoms were rapidly relieved and chemical signs corrected by introducing frequent protein-rich meals and night-time feedings of suspensions of uncooked corn (maize) starch. The discovery of oligosymptomatic and asymptomatic siblings suggests that there are more persons with undiagnosed hepatic glycogen synthase deficiency.