BACKGROUND: No exhaustive data are available on the in vivo biological effects of pegfilgrastim utilized in dose-dense chemotherapy (CT). The cytokinetic effects exerted in a multicyclic CT program by this cytokine on CD34+/38+ peripheral blood (PB) progenitor cells was the focus of this study. PATIENTS AND METHODS: PB samples from 19 breast cancer patients treated with 4 courses of docetaxel and epirubicin followed by pegfilgrastim were studied. The absolute number of CD34+/38+ circulating progenitor cells (CPCs) along with the percentage undergoing GO/G1, S and G2-M phases of the cell cycle or showing apoptotic features, were evaluated at baseline, after the first and before the fourth CT course using a dedicated flow cytometric technique. RESULTS AND CONCLUSION: Pegfilgrastim, after CT, exerted stimulatory effects on the cell cycle status of PB CD34+/38+ CPCs, at the same time protecting them from apoptosis. This was particularly evident 7 days after administration and tended to decrease one week later, without additional cytokinetic changes during the subsequent CT courses.