A study of 2-deoxyglucose transport into polymorphonuclear leukocytes (PMN) was performed in three patients with glycogen storage disease (GSD) type Ib. The rate of 2-deoxyglucose transport into GSD Ib PMN was 30% of that of cells of normal controls. In GSD Ib lymphocytes, transport was normal. Km for 2-deoxyglucose in the PMN of one patient was within the normal range. The reduced transport was not due to the elevation in Km for 2-deoxyglucose nor to the decreased rate of phosphorylation of 2-deoxyglucose. The striking limitation of glucose transport across the cell membrane may account for the impairment of leukocyte function which is characteristic of GSD Ib.