Cyclic neutropenia developed in identical twin girls. The onset of neutropenia in these children occurred three years apart. Neutrophil cycling diminished, and symptoms decreased in the initially affected twin during a five-year follow-up. Some cases of cyclic neutropenia may be genetically determined; however, the onset and clinical manifestations may be modified by other internal and external factors. There may also be a prodromal period during which neutrophils cycle, but the patient is neither neutropenic nor symptomatic.