Neutropenia is common in neonates with sepsis and in those born to women with pregnancy-induced hypertension. Neutropenia has not previously been described, however, as a result of the twin-twin transfusion syndrome. We observed neutropenia of 4 to 8 days' duration in each of five "donor" (anemic) twins affected with the twin-twin transfusion syndrome. No evidence of infection was observed. Like neutropenia of pregnancy-induced hypertension, no left shift was seen. Neutrophil kinetic studies were performed on one of the neutropenic patients. Assessment of the marginal, storage, proliferative, and progenitor cell pools indicated that neutropenia resulted from diminished neutrophil production.