The application of recombinant colony stimulating factors for chemotherapy induced granulocytopenia is becoming common in clinical oncology. Here we report on localized cutaneous side effects after subcutaneous administration of recombinant human granulocyte-macrophage colony-stimulating factor (rh GM-CSF) in 11 patients with breast cancer receiving cytostatic treatment. Seven patients suffering from inflammatory breast cancer received cytostatic chemotherapy with mitoxantrone/cyclophosphamide, whereas four patients suffering from noninflammatory breast cancer received high-dose epirubicin/cyclophosphamide, respectively. rh GM-CSF was applicated subcutaneously in a dose of 5 micrograms/kg/d for at least ten days. In all patients, sharply demarked, maculous itching and burning erythemas restricted to the injection sites occurred after three to four injections of rh GM-CSF. These eruptions cleared within 2 to 3 weeks, but reappeared after reexposure to rh GM-CSF. In contrast to previous sporadic reports, no generalized erythemas were observed. Because of this unexpected and subjectively intolerable side effect, rh GM-CSF administration had to be interrupted in all patients. Histopathological findings revealed skin infiltration with lymphocytes, monocytes/macrophages, neutrophils, and occasionally eosinophils, respectively. Since GM-CSF is known to alter immune functions, it seems likely that the eruptions were at least in part due to local immune reactions.