GM-CSF is a major regulator of myelopoiesis. Recombinant human GM-CSF (250 micrograms/m2 per day i.v.) was used prior to chemotherapy ("3 + 7" scheme) to recruit leukemic blasts in vivo (de novo AML patients, n = 20) into the chemotherapy sensitive phases of the cell cycle. The stimulatory effect of GM-CSF on peripheral blood AML blasts was associated with a rapid redistribution of leukemic blood cells and with an increase in "S-phase positive" cells. Standard induction chemotherapy ("3 + 7") following GM-CSF induced complete aplasia in 19/20 patients. In the same patients, rhGM-CSF (given after chemotherapy) was found to shorten the time of complete aplasia compared to historical controls whereas post-chemotherapy- and follow-up data suggest no significant differences for CCR and survival. Together, our studies show that GM-CSF can safely be administered to AML patients in combination with induction chemotherapy to recruit leukemic cells into the cell cycle.