The peripheral blood morphologic findings in 17 patients with cancer who had received high-dose cytotoxic chemotherapy followed by recombinant human-granulocyte colony-stimulating factor (rh-GCSF) were reviewed and compared with a control group of patients who received only high-dose chemotherapy. Both groups showed dysmyelopoiesis (abnormal granulation and nuclear lobulation) in the granulocytic series during the period of bone marrow recovery that followed the cytotoxic chemotherapy. Most of these morphologic abnormalities were more prominent in the rh-GCSF-treated group. Monocytic cells in both groups showed prominent vacuolation and immature nuclei. The percentages and absolute numbers of large granular lymphocytes were increased in the rh-GCSF group compared with the control group. No quantitative or qualitative abnormalities of eosinophilic or basophilic granulocytes were detected in either group. Both groups showed nonspecific red blood cell abnormalities, and large platelets were present in half of the control group smears. This report provides the first detailed peripheral blood morphologic description in patients treated with rh-GCSF and high-dose chemotherapy.