The effect of supplementing induction chemotherapy with recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) was studied in a randomized trial of 18 patients with acute myeloid leukemia (AML). Ten patients received rhGM-CSF, starting on day one to three before chemotherapy and continued for a maximum of 21 days after the start of induction treatment. Unexpected adverse effects of rhGM-CSF and chemotherapy combination included a transient decline in plasma coagulation factors II, VII, and X (5 of 5 patients) and an increased transcapillary escape rate of albumin (in 3 of 3 patients tested). The decline in coagulation factors was prevented in subsequent patients by prophylactic treatment with vitamin K. Although the small number of patients studied may not allow a definite conclusion, caution with regard to liver function should be shown in combining rhGM-CSF with intensive chemotherapy.