GM-CSF decreases the hematopoietic toxicity of both nonablative and marrow ablative intensive chemotherapy regimens. Data are limited at this time, particularly in regard to optimal schedules and GM-CSF dosing regimens with particular chemotherapy programs. The importance of GM-CSF scheduling in relationship to chemotherapy is becoming more clear. The dose-intensive regimens supported by GM-CSF produce a high complete remission rate in several types of malignancy. Evidence that these remissions will be durable is still lacking. Future directions will include filling some of these voids in our knowledge, as well as exploring various cytokine combinations for improved hematopoietic recovery and other chemotherapeutic regimens for improved antitumor effect. The near complete cytoreduction produced by these dose-intensive regimens may also create a better setting for attempts at therapeutic immunomodulation. Yet another, but more difficult, task will be to identify those situations where progenitor cell replacement is beneficial and appropriate.