Bone pain is a common side effect of treatment with filgrastim. Pegfilgrastim is a pegylated long-acting analogue of filgrastim that is administered once per chemotherapy cycle. The profile of prospectively defined, patient-reported bone pain judged by the investigators as related to study drug was analyzed retrospectively for each drug using data from two comparable phase III trials. These multicenter, randomized, double-blind, noninferiority trials compared once-per-cycle pegfilgrastim (6 mg, study 1 or 100 microg/kg, study 2) to daily filgrastim 5 microg/kg in patients with stage II-IV breast cancer undergoing multiple cycles of myelosuppressive chemotherapy (doxorubicin/docetaxel). Subcutaneous once-per-cycle pegfilgrastim 6-mg and 100-microg/kg doses were administered to 76 and 150 patients, respectively; subcutaneous daily filgrastim 5 microg/kg was administered to a total of 227 patients. Because bone pain in study 1 was higher (P = 0.044) in every cycle compared with study 2, all analyses were performed separately for each study. No statistically significant differences in incidence, severity, or duration were observed between patients receiving either once-per-cycle pegfilgrastim or daily filgrastim in either study. Bone pain incidence and severity were significantly greater (P < 0.001) in cycle 1 of both studies compared with later cycles. Among patients with bone pain, a trend towards earlier onset with pegfilgrastim was observed but was not associated with increased bone pain severity or duration. In patients who received a fixed 6-mg dose of pegfilgrastim, the overall bone pain incidence was similar when analyzed by body weight (< 60 kg, 60-100 kg, > 100 kg). No patients were withdrawn from either study for bone pain.