The absence of an effective therapy for most patients with leukemia who relapse after allogeneic BMT has generated interest in new strategies. We present our experience on the use of filgrastim 5 micrograms/kg/day s.c., in four patients with leukemia (three with AML and one with CLL) who relapsed after allogeneic transplantation. One patient with AML achieved CR after 55 days of treatment. No response was observed in the remaining three. The patient who responded developed extensive chronic GVHD but relapsed 10 months later. In one of the unresponsive patients a dramatic increase in bone marrow infiltration and WBC count followed administration of filgrastim. We conclude that filgrastim can occasionally induce CR in leukemic patients who relapse after BMT.