We present a patient with refractory anemia (RA) who developed Sweet's syndrome during the treatment of recombinant human granulocyte colony-stimulating factor (rhG-CSF). A 30-year-old man was admitted to the hospital for evaluation of anemia. He was diagnosed as MDS (RA). As a phase II study in MDS, rhG-CSF therapy was begun. Fever associated with cutaneous lesion developed over the left shoulder. Antibiotics showed no effects. Skin biopsy revealed Sweet's syndrome. This skin lesion disappeared thoroughly with discontinuance of G-CSF and administration of prednisolone. To examine whether Sweet's syndrome was related to the G-CSF therapy, we analyzed the effect of G-CSF on the function of patient's neutrophils. However, the function of patient's neutrophils was not activated by G-CSF administration.