Despite numerous attempts to increase the neutrophil count of infants with alloimmune neonatal neutropenia, no therapy has been consistently effective. We describe two infants with alloimmune neutropenia who had a rapid and prolonged increase in neutrophil number after treatment with granulocyte colony-stimulating factor (G-CSF). Patient 1 had antibody directed against the neutrophil antigen NA2. He received three daily doses of G-CSF, and within 2 days his neutrophil count increased from 0.350 x 10(9) to 3.584 x 10(9)/L (350 to 3584/mm3). Despite cessation of treatment the neutrophil count remained in the normal range. Patient 2 had antibody to the neutrophil antigen NA1, and received six daily doses of G-CSF. Within 4 days his neutrophil count increased from 0.477 x 10(9) to 4.320 x 10(9)/L (477 to 4320/mm3) and remained in the normal range for 11 days after the last dose of G-CSF. We recommend that treatment with G-CSF be considered for selected infants with alloimmune neutropenia.