Five children with glycogen storage disease type Ib (glycogenosis Ib), a metabolic defect associated with neutropenia and impairment of neutrophil function, were treated with granulocyte colony stimulating factor, a haemopoietic growth factor that induces a significant increase in polymorphonuclear leucocyte count in vivo. Recurrent bacterial or fungal infections were recorded in the three patients who had very low polymorphonuclear leucocyte counts. Experience at this centre indicates that the absolute number of polymorphonuclear leucocytes is more important than the alteration of their metabolic function in determining the susceptibility of the patients to infection. Granulocyte colony stimulating factor was effective at increasing polymorphonuclear leucocyte numbers (and restored function to some extent) in patients with glycogenosis Ib. This drug might be beneficial in cases of severe infection, in glycogenosis Ib patients with neutropenia.