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Purpose: To evaluate safety of balugrastim, a recombinant human serum albumin and granulocyte colony-stimulating factor (G-CSF), administered over a range of therapeutic doses in women with breast cancer receiving doxorubicin plus docetaxel chemotherapy. Methods: The phase I, sequential dose-escalation first segment compared subcutaneous balugrastim 50, 150, 300, and 450 µg/kg during chemotherapy cycles 0-2. The randomized (2:2:1), open-label, phase IIa second segment compared balugrastim 300 or 450 µg/kg with pegfilgrastim 6 mg during chemotherapy cycles 1 and 2. Results: In the phase I segment, balugrastim was escalated to 450 µg/kg in 13 patients without dose-limiting toxicity. Three (9.7 %) of the 31 adverse events (AEs) reported in nine patients were grade 3 (agranulocytosis, vomiting, hypertension); none was grade 4. In the open-label phase IIa segment ( N = 51), the majority of the 64 AEs reported in 31 (75.6 %) balugrastim-treated patients were grade 1 (59.4 %), with 39.1 % grade 2, 1.6 % grade 3 (one AE of vomiting), and none grade 4. Of the 16 AEs reported in seven (70.0 %) pegfilgrastim-treated patients, 87.5 % were grade 1, 6.3 % were grade 2, 6.3 % were grade 3 (one AE of thrombocytopenia), and none were grade 4. Overall, there were six bone pain AEs reported, one in the balugrastim 300 µg/kg group and five in the balugrastim 450 µg/kg group. No AEs in either study necessitated treatment interruption/discontinuation. The incidence and duration of grade 3-4 neutropenia were similar between balugrastim- and pegfilgrastim-treated patients. Conclusions: Balugrastim was well tolerated in this small population of breast cancer patients. [ABSTRACT FROM AUTHOR]