The complication of secondary myelodysplastic syndrome (sMDS) during the course of multiple myeloma (MM) has been recognized for more than a decade. sMDS occurs years after MM diagnosis, and typically, at sMDS presentation the MM is stable or inactive. We report a 56-year-old patient, who developed sMDS 15 years following the diagnosis of IgG-lambda MM, which had been completely stable for 13 years. However, very soon after sMDS was diagnosed, the MM relapsed and required combination chemotherapy. The first cycle of vincristine, adriamycin and dexamethasone (VAD) resulted in severe neutropenia and sepsis, which was treated with antibiotics and recombinant human granulocyte-macrophage colony-stimulating factor (rHuGM-CSF). Two weeks after GM-CSF administration a transformation to acute myeloblastic leukemia was observed. The relation between GM-CSF and the leukemic transformation is discussed and the possible contribution of the cytokine to the stimulation of this complication is emphasized.