Cyclic neutropenia is a benign, hematologic disorder characterized by recurrent episodes of severe neutropenia at 21 day intervals. There are associated cyclical variations in other blood cells. Patients with this disease have malaise, stomatitis, cervical lymphadenopathy and fever during the recurrent neutropenic periods. The exact cause of cyclic neutropenia is unknown. About one third of human cases appear to be inherited in an autosomal dominant pattern. In the other cases, the disease appears to arise spontaneously with symptoms usually beginning in infancy or early childhood. In adult patients, the disease may be acquired and occur in association with a clonal proliferation of large granular lymphocytes. Clinical studies in man and investigations in grey collie dogs, which have a very similar disease, strongly suggest that cyclic neutropenia is due to an abnormality in the regulation of early hematopoietic precursor cells. Therapy for cyclic neutropenia involves local and symptomatic therapy for the recurrent mouth ulcers and pharyngitis, and antibiotics for episodes of sinusitis, pneumonia, peritonitis, or bacteremia. Therapy with glucocorticosteroids, androgens, and plasmapheresis has been efficacious in a few adult patients, but no therapy has been proven to alter the cycling of blood counts in children. Despite their repetitive illnesses, patients with cyclic neutropenia grow and develop normally. With the help of attentive physicians and dentists, their quality of life and life expectancy are good. Current research on hematopoietic growth factors offers promise of new approaches to therapy.