The purpose of this study is to provide data on the safety and efficacy of pegfilgrastim when administered on the same day versus the next day of chemotherapy, as measured by the duration of grade 4 neutropenia.

Inclusion Criteria: - Breast cancer, previously untreated Exclusion Criteria: - Active infection requiring treatment with systemic (intravenous or oral) anti-infectives (antibiotic, antifungal, antiviral) within 72 hours of chemotherapy - Prior malignancy within the last 5 years, with the exception of surgically cured basal/squamous skin cell carcinoma, ductal carcinoma in situ of the breast and/or carcinoma of the cervix in situ - History of impaired cardiac status (e.g., severe heart disease, cardiomyopathy or congestive heart failure) - Major surgery within 2 weeks prior to randomization - Previous exposure to cytokines within two weeks prior - Concurrent use of other investigational agents