Granulocyte colony-stimulating factor (G-CSF) enhances the differentiation of acute promyelocytic leukemia (APL) cells induced by all-trans retinoic acid (ATRA) in vitro. Accordingly, we initiated a pilot study on G-CSF in APL patients who developed neutropenia and severe infection during remission induction therapy with ATRA. Seven out of nine treated patients displayed a marked increase in granulocyte counts without leukemic regrowth, and two displayed a dramatic decrease in leukemic blasts. However, leukemic regrowth occurred in two patients under treatment for post-ATRA relapse. Our findings suggest that administration of G-CSF combined with ATRA can improve the hematological state in APL patients not previously receiving ATRA therapy.