Human cyclic neutropenia is a relatively rare disorder of unknown etiology. Study of patients and animals with the disorder has led to important information regarding the differentiation of blood cells and control mechanisms of hematopoietic regulation. It has a world-wide distribution, occurs in both sexes, and, in about one-fourth of the patients, a family history has been obtained. While usually benign, deaths from overwhelming infections occur. In addition to cycling of neutrophils, in the majority of cases the monocytes cycle and in about one-fifth of the cases eosinophils are elevated. In a small number of patients, cycling of platelets and reticulocytes occurs. Cycles of colony stimulating factor are present. Cycles of bone marrow cells are easily demonstrable. The recent transfer of human cyclic neutropenia following allogenic bone marrow grafting confirms the hypothesis that the disorder is of bone marrow origin. The following subjects are covered in this review article: A. Definition, history, and incidence; B. Etiology, geographic distribution, mode of transmission; C. Symptoms, physical signs, diagnosis, clinical course; D. Clinical laboratory studies; E. Experimental studies; F. Prognosis; G. Treatment. It is felt that human cyclic neutropenia represents a heterogeneous group of disorders and that much remains to be learned about its cause(s).