In order to investigate the safety and efficacy of stem cell mobilization in chronic myeloid leukemia patients under imatinib therapy we treated 10 such patients with granulocyte colony-stimulating factor. We observed that none of the patients developed progressive disease under this treatment. Instead, sufficient CD34+ apheresis could be performed in 7 patients and, as assessed by nested reverse transcriptase polymerase chain reaction (RT-PCR), bcr/abl-negative stem cell products could be generated in 3 patients. Interestingly, in 3 other patients with bcr/abl-positivity in 1st round RT-PCR of peripheral leukocytes, bcr/abl transcripts in stem cell products could only be detected by nested RT-PCR.