Chemotherapy (CT) induced critical cytopenia remains as the major dose limiting problem in the treatment of acute leukemias. This is especially true in patients at high risk of early death due to high intensity chemotherapy for relapse or to higher age. In an attempt to reduce the phase of risk we administered human recombinant GM-CSF to 23 patients at a median age of 65 (range 17-84) years including 8 acute myelogenous (AML) and 5 lymphoblastic (ALL) leukemias after early or second relapses and 10 patients of 65 years and older in primary induction treatment for AML. 3 patients with AML had prior bone marrow transplantation. 4 AML's were secondary to tumor chemotherapy (2) or myelodysplastic syndrome (2). The study was part of chemotherapeutic phase II and III studies using TAD9 for primary induction chemotherapy and S-HAM for relapses. Starting on day 4 after the end of intensive CT GM-CSF 250 micrograms/m2/day was given by continuous i.v. infusion and after recovery of neutrophils was deescalated in two 4 day steps and discontinued. 12/23 patients achieved a complete remission (CR) and median age of responders is 61 (range 17-84) years. In the group of the non-transplanted patients the median recovery time of neutrophils is reduced by one week when compared to controls receiving the same chemotherapy (p = 0.002). We observed a leukemic regrowth in 3 patients of 61, 70, and 78 years, 2 of whom having secondary AML. After discontinuation of GM-CSF the regrowth of blasts was reversible in 1 patient and continued unaffectedly in its kinetics in 2 patients.(ABSTRACT TRUNCATED AT 250 WORDS)