We have identified two unrelated girls with chronic neutropenia [absolute neutrophil counts (ANC) 10–870 and 10–940/μL in patients 1 and 2, respectively] and severe defect in superoxide anion generation by granulocytes. Formyl-methionyl-leucyl-phenylalanine-induced superoxide release was 1.2 ± 0.9 and 1.9 ± 1.9% (mean ± SEM,n= 3) of normal controls‘, mean value in patients 1 and 2, respectively. However, granulocytes from both patients released a normal amount of superoxide upon stimulation with phorbol myristate acetate. Patient 2 exhibited characteristic features of Duane syndrome, a rare disorder of eye movement. Treatment of the patients with recombinant granulocyte colony-stimulating factor led to significant clinical improvements and reduction of infectious complications and to increases in the ANC, to 400–2100/μL in patient 1 and to 500–3000/μL in patient 2. Treatment with 5 μg/kg/d resulted in increased intracellular killing of opsonizedStaphylococcus aureusby granulocytes and an enhancement of superoxide release upon stimulation with formyl-methionyl-leucyl-phenylalanine in both patients up to 11.1 ± 6.0 and 13.5 ± 7.0% (mean ± SEM,n= 5) of normal controls’, mean value in patient 1 and patient 2, respectively. These data suggested that recombinant human granulocyte colony-stimulating factor treatment enhanced resistance to bacterial infection by stimulation of superoxide generation and increasing the bactericidal capacity of peripheral blood granulocytes.