As part of a multicenter trial 12 patients with myelodysplastic syndromes (MDS) were treated with 14-day-cycles of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF; 250 μg/m2 day s.c). In addition, all patients received 20mg/m2/ day s.c. cytosine-arabinoside (Ara-C) 12 h after GM-CSF except for patients suffering from refractory anemia (RA) according to FAB classification. Courses were repeated after 4 weeks. In 11 evaluable patients, results according to FAB-classified MDS were a follows: RA, 1/2 response (R), 1/2 stable disease (SD); RAEB, 2/3 R, 1/3 SD; RAEB-T, 1/6 CR, 1/6 PR, 2/6 R, 2/6 progression; CMML, 1/2 SD. In 2 patients with RAEB-T, overt acute myeloid leukemia was observed 2 and 10 weeks after initiation of treatment. With few exceptions, treatment resulted in a prompt increase in granulocytes and eosinophiles. This was associated with improvement of infectious complications. Increases in red cells and platelets occurred variably and was apparently associated with responses of the underlying disease. Dose limiting side effects consisted of fever, severe fatigue and dolent local reactions at the site of GM-CSF injection. In addition, nausea and diarrhoea occurred frequently. Less often, respiratory and cardiovascular side effects were encountered. In summary, GM-CSF ± Ara-C in MDS results in objective remission with manageable toxicity. Conceivably, this regimen will serve as a base for future treatment strategies against MD