Eleven adult patients with chronic neutropenia have been treated with recombinant human granulocyte colony-stimulating factor (G-CSF). Seven patients had idiopathic sporadic neutropenia, 2 idiopathic congenital neutropenia and 2 cyclic neutropenia. Treatment was started with 3 micrograms/kg s.c. daily and was modified according to response. All patients had a rapid increase of neutrophils. The cycle length shortened from 21 to 14 days in patients with congenital cyclic neutropenia. Doses required for maintenance ranged from 0.1 to 8 micrograms/kg. One patient with idiopathic sporadic neutropenia recovered after 151 days with an absolute neutrophil count (ANC) of > 2000/microliters and had no need for further treatment. The overall efficacy of the treatment was good with abrogation of severe infections. Treatment has been continuously given to a maximum of 4.5 years. No loss of efficacy was observed during long-term treatment. There was no evidence for lineage drain or stem cell exhaustion with prolonged treatment. Cytogenetic analyses in seven of eleven patients did not reveal the development of abnormal cell-clones. G-CSF is safe and efficient in long-term treatment of adult patients with chronic neutropenias.