The aim of this study was to evaluate the effect of dose-dense adjuvant chemotherapy regimens with peg-filgrastim support on the phenotype of peripheral blood leukocytes in breast cancer patients. We evaluated the leukocyte phenotype of 14 patients aged 46-67 years undergoing 4 courses of chemotherapy with either epirubucin/cyclophosphamide (n=7) or 5-fluorouracil/epirubucin/cyclophosphamide (n=7) followed by 4 courses of taxol supported by peg-filgrastim (6 mg) administered 72 h after each chemotherapy course. The overall leukocyte number significantly increased from the first treatment course, while total lymphocytes tended to decrease with a negative peak following the 6th course (p=0.03). B (CD19+, CD20+) and early B lymphocyte subsets (CD20+/CD38+) significantly decreased during treatment (p<0.05), while T lymphocyte subsets did not show significant changes, except a decrease in T helper (CD4+) cells. Immature T lymphocytes (CD4+/CD8+ subset), dendritic cells (CD11c+) and NK cells (CD56+) increased with respect to the baseline. Our results suggest that dose-dense chemotherapy programs with the support of peg-filgrastim did not significantly impair the immune system of breast cancer patients and allowed for a rapid restoration of most immune competent cells. These observations may have important clinical implications with a view to vaccination or other immunotherapeutic approaches to solid tumours.