Neutropenia is a severe adverse effect that can occur when treating patients with imatinib mesylate for advanced-stage chronic myelogenous leukaemia (CML). Therefore, we evaluated in vitro the combined effect of imatinib and granulocyte colony-stimulating factor (G-CSF) on proliferation and apoptosis of Bcr-Abl-expressing leukaemic cells to infer the safety of G-CSF administration. In KU812 and K562 cell lines, G-CSF neither stimulated their proliferation nor abolished the suppressive effect of imatinib. However, it stimulated the proliferation of blast cells in 2 out of the 5 cases with advanced-stage CML. These in vitro studies appear to provide data for the decision of G-CSF administration in combination with imatinib in the treatment of neutropenic patients with advanced-stage CML.