Cyclic neutropenia in identical twins.

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.