The aim of this study was to evaluate the efficacy, safety and toxicity of short-term priming with recombinant human granulocyte colony-stimulating factor (rhG-CSF) immediately after diagnosis but before combination chemotherapy (CHOP) for non-Hodgkin's lymphomas. Of fourteen patients entering the study, seven received five days subcutaneous injection of rhG-CSF (5 micrograms/kg/day) before CHOP (CSF-group), and seven were treated with CHOP alone (control group). Blood samples were studied before and on days 1-5 during rhG-CSF priming as well as twice weekly after treatment. The number of blood and bone marrow progenitors was identified by clonogenic growth day 7, 14 and 21 of GM-CFU in semisolid medium. Blood absolute neutrophil counts increased in all rhG-CSF primed patients. The expansion of marrow myelopoiesis resulted in increased myeloid:erythroid ratios, increased bone marrow cellularity and increased numbers of myeloid progenitors both in the blood as well as the marrow. Chemotherapy induced neutropenia developed on day 9-12 in all patients independent of myeloid growth factor priming. However, neutropenia appeared earlier in the cytokine primed group (P = .0038). Five patients in the CSF-group and three patients in the control group were hospitalized with neutropenic fever, and septicemia was documented in three patients in the CSF-group. RhG-CSF induced expansion of myelopoiesis immediately before combination chemotherapy mobilized sufficient number of blood progenitors for apheresis but did not result in reduction of duration and degree of neutropenia in patients with newly diagnosed non-Hodgkin's lymphoma. Although the small number of patients prevents drawing definite conclusions, this time schedule for priming should be used with caution in the future due to an increased risk of hematologic toxicity.