OBJECTIVE Recombinant human granulocyte colony-stimulating factor (rhG-CSF) is effective in the prophylaxis and management of chemotherapy-induced neutropenia, but requires daily administration because of its short half-life. Pegylated rhG-CSF (PEG-rhG-CSF) is a long-acting reagent that permits less frequent injection. This study was to evaluate the safety and tolerance of PEG-rhG-CSF in Chinese patients, and to explore its efficacy of enhancing absolute neutrophil count (ANC) and CD34+ cell count in peripheral blood. METHODS Naïve non-small lung cancer or breast cancer patients with normal bone marrow function were eligible for this open-labeled, dose-escalation trial. All patients received 2 cycles of chemotherapy of identical regimen. In cycle 1, rhG-CSF (150 microg/day) was administrated in case of febrile neutropenia or grade 4 neutropenia; in cycle 2, patients received a single injection of PEG-rhG-CSF (30 microg/kg, 60 microg/kg, 100 microg/kg, or 200 microg/kg) 48 h after administration of paclitaxel and carboplatin. RESULTS All the 16 patients enrolled (4 in each dose group) were evaluable for safety and efficacy of PEG-rhG-CSF. Main adverse events related to PEG-rhG-CSF were musculoskeletal pain or arthralgia (13/16), fatigue (10/16), dizziness (2/16), and injection-site pain (1/16). All adverse events were mild to moderate, and most of them were reversible without treatment. PEG-rhG-CSF enhanced ANC in a dose-dependent manner to some extent, and PEG-rhG-CSF at 60 microg/kg or higher doses prevented chemotherapy-induced neutropenia with sustained effect; CD34+ cells in peripheral blood were also increased. CONCLUSIONS PEG-rhG-CSF is well tolerated, with no serious adverse event in this trial. The recommended dose of PEG-rhG-CSF for phase II trial is 100 microg/kg because of its adequate efficacy and less adverse events than those of 200 microg/kg.