The fasting glucagon test of 40 patients with hepatic glycogen storage disease (type I, 13 patients; type Ib, 5 patients; type III, 12 patients; type IX (phosphorylase kinase deficiency), 10 patients) has been reviewed. In all patients with types Ib and III the blood glucose level rose less than 1 mmol/1 but in types 1 and IX there was wide variation in response. Blood lactate levels were greater than 2.4 mmol/1 in all patients with type I and Ib 120 minutes after the administration of glucagon but were lower than 2.4 mmol/1 in types III and IX. The value of this test for screening was analysed using several criteria. From the data a simplified scheme for the investigation of patients with suspected hepatic glycogen storage disease is proposed.