A 34-year-old female with cyclic neutropenia is reported. Family studies showed that her three sons and her mother were also involved. Oscillations in the blood neutrophil counts were almost regular, with a periodicity of 21 days. Numbers of colony-forming unit--granulocyte macrophage (CFU-GM) formed from the bone marrow cells of normal volunteers co-cultured with the patient's serum or mononuclear cell-conditioned medium (MNC-CM) were examined. Her serum prepared during the neutropenic phase inhibited the growth of CFU-GM, while her MNC-CM stimulated it. Human granulocyte colony-stimulating factor (hG-CSF) level in her serum was persistently high, with the peak occurring during the neutropenic phase. These results suggest that some inhibitory factors in the serum may be pathophysiologically important for cyclic neutropenia. To control infections, a pharmacological dose of hG-CSF was administered for 7 days around the early neutropenic phase. Her peripheral neutrophil counts oscillated from 1,200/mm3 to 17,000/mm3 with G-CSF, and from 150/mm3 to 1,800/mm3 without G-CSF.