In a preterm infant, small-for-gestational age, neutropenia persisted from the birth, and periomphalitis symptoms appeared at 23 days of age. Upon administration of granulocyte colony-stimulating factor, the neutrophil count increased, and periomphalitis was recovered. At the same time, the umbilical cord separated at 44 days of age. The human neutrophil antigen (HNA) of the infant was 1a/1b, and antineutrophil antibody for HNA-1b was present in both serums of the mother and the infant at birth, indicating a diagnosis of alloimmune neutropenia. This case demonstrated a definite relationship between the number of neutrophils and the separation of umbilical cord.