The effect of recombinant human granulocyte colony-stimulating factor (rhG-CSF) administration at 10 microg/kg/day for 15-30 min over 3 consecutive days in 3 preterm neonates with allo-immune neonatal neutropenia (ANN) is reported. Patients 1 and 2 had antibodies against antigen NA2, while patient 3 had antibodies against NA1. All neonates developed a rapid increase in absolute neutrophil count which reached the normal range within 48 h (from 75-640/mm(3) to 2,520-4,700/mm(3)). However, 23-25 days later, all 3 neonates relapsed into a second phase of severe neutropenia (408-870/mm(3)). Antibodies against neutrophil antigens were still positive during this period. This second-phase neutropenia persisted for 20-30 days and resolved spontaneously. It may be possible that when rhG-CSF is administered within a short time after birth in neonates with ANN, its effect is exhausted before the concentration of circulating antibodies decreases, with the result that a second phase of neutropenia can be expected.