Severe congenital neutropenia is a disorder of myelopoiesis characterized by severe neutropenia secondary to either a maturational arrest of myelopoiesis at the level of promyelocytes (Kostmann-Syndrome; SCN) or regular cyclic fluctuations in the number of blood neutrophils with a median ANC below 500/microliter (cyclic neutropenia). We have treated 32 patients with SCN and 4 patients with cyclic neutropenia. Thirty of 32 patients with SCN and all 4 patients with SCN responded to r-met HuG-CSF treatment with an increase of the median ANC to above 1000/microliter. The doses needed to achieve and maintain the response varied between 0.8 and 120 micrograms/kg/d. Long-term treatment did not exhaust the myelopoiesis: The mean ANC remained stable up to 5 years of treatment. The increase in ANC was associated with dramatic clinical responses: significant reduction of severe bacterial infections, reduction of intravenous antibiotic treatment episodes, and reduction of hospitalizations. No severe bacterial infections occurred in any of the r-met HuG-CSF responders during long-term treatment. Severe adverse event, most likely associated with the underlying disease, included the development of MDS/Leukemia in two patients, and osteopenia/osteoporosis in 12 patients. These results demonstrate the beneficial effects of r-met HuG-CSF treatment in severe congenital neutropenia patients.