The availability of rh GM-CSF has allowed the in vivo treatment of patients with cytopenia. Therefore a phase I/II trial was initiated to study the effect of rh GM-CSF in patients with myelodysplastic syndromes who were not eligible for other kinds of therapy. rh GM-CSF has been tested in 10 patients in doses from 15 micrograms/m2 to 150 micrograms/m2 given intravenously over 8 hours for a cycle of 7 days followed by an interval of 14 days and a second 7-day treatment course. A dose dependent increase in leukocyte count was observed in 9 out of 10 patients. No change in reticulocyte numbers was seen and only one patient experienced an increase in platelet count. Toxicity mainly consisted of mild local phlebitis at the site of infusion and sternal pain after bolus injection. An increase in blast cell counts in some patients necessitated the start of low dose Ara-C therapy.