Recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) is widely used in the treatment or prevention of neutropenia induced by cytostatic regimens. Recent studies with this cytokine have shown several local and/or systemic side effects. We herein report on four patients with different tumor entities receiving GM-CSF as a part of their intensified cytostatic regimen. All four patients developed immune phenomena (sicca syndrome, seropositive arthralgia, hyperthyroidism, and pneumonitis, respectively) during or after subcutaneous treatment with GM-CSF. Pathologic alterations in immunologic serum parameters as well as histopathologic findings accompanied the clinical symptoms. These observations suggest that the therapeutic application of GM-CSF might be involved in the clinical emergence of autoimmune diseases.