Our observations suggest that bone marrow examinations including cytogenetic analysis should be carried out on a regular basis in patients with glycogen storage disease type Ib who are on long-term treatment with granulocyte colony-stimulating factor for severe neutropenia, since this treatment might also contribute to an increased risk for acute myeloid leukemia or myelodysplastic syndromes. The gene discussed is CSF3; the disease is acute myeloid leukemia.