A 12-month-old boy with Kostmann's syndrome was admitted with cavitary pulmonary disease. He had also had bacterial conjunctivitis, periorbital cellulitis, pneumonitis, and otitis media since the age of 10 days. His umbilical cord had not fallen off until he was 3 weeks old. Neutropenia was diagnosed at 4 weeks of age. Antineutrophil antibody studies were negative. A bone marrow aspirate showed granulocytic hypoplasia and a maturation arrest at the promyelocyte stage. Hematopoietic cell culture showed normal numbers of colony-forming units-granulocyte macrophage. Serum granulocyte-macrophage colony-stimulating factor level, was 0.24 ng/mL (normal, greater than 0.05 ng/mL). Serum granulocyte colony-stimulating factor levels, measured by enzyme immunoassay, were undetectable. The patient was successfully treated with filgrastim (granulocyte colony-stimulating factor), with an increase in the absolute neutrophil count to 10.0 x 10(9)/L. Thus, our case of Kostmann's syndrome appears to represent a defect in regulation or production of granulocyte colony-stimulating factor.