Clinical value of granulocyte colony-stimulating factor (G-CSF) for autoimmune neutropenia (AIN) has not been well established. We experienced an adult case of AIN which showed an excellent response to G-CSF. A 75-year-old female was admitted with high-grade fever. Her neutrophil count was remarkably low (neutrophil 0.09 x 10(9)/l). Antigranulocyte autoantibody was demonstrated in her serum by an immunofluorescence method and she was diagnosed as AIN. Administration of G-CSF (filgrastim 5 microgram/kg) gave a rapid increase of neutrophils (from 0.11 x 10(9)/l to 2.10 x 10(9)/l on the second day), which has enabled us to preserve the use of G-CSF for emergency, that is, for overt serious infection.