The primary objective of this study was to compare the toxicity and hemopoietic effects of s.c. and i.v. recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) in patients with primary myelodysplasia. Twenty patients were treated in this phase I crossover-designed study. In three groups of patients, the dose of rhGM-CSF was escalated from 60 to 250 micrograms/m2/day. Each patient was to receive 2 weeks of i.v. (daily 2-h infusion) and s.c. (twice daily) rhGM-CSF separated by a 2-week rest period. The decision to start with i.v. or s.c. administration was by random selection. Toxicity was comparable between i.v. and s.c. administration. At the highest dose level, 63% (five of eight) of the patients developed moderate to severe toxicity. Increases in the absolute neutrophil count showed a dose-response relationship and were more pronounced with s.c. than i.v. administration. Failure to grow in vitro granulocyte-macrophage colonies or a hypocellular marrow (less than or equal to 30%) predicted for a poor response to therapy. No patient had a platelet or a reticulocyte response. No patient progressed to acute leukemia. Compared to a 2-h infusion of i.v. rhGM-CSF, s.c. administration is more myelostimulatory without an increase in toxicity.