Thirty-four patients diagnosed with breast cancer were included in a prospective study evaluating the bone marrow (BM) CD34+/CD71- cell content, as a predictive parameter of the CD34+ cell mobilization after rG-CSF administration. Analysis of the concentration of medullary CD34+/CD71- cells before priming schedules was significantly related with the collection of CD34+ cells in apheresis day 1 (P = 0.03, r = 0.36), apheresis day 1 + day 2 (P = 0.01, r = 0.42) or the total CD34+ cells collected (P = 0.005, r = 0.47). A BM CD34+/CD71- cell concentration greater than or less than a cut-off value of 30/microl was significantly associated with the yield of CD34+ cells collected by cytapheresis procedures (mean values 3.12 x 10(6)/kg, and 2.19 x 10(6)/kg, respectively, P = 0.013). These results suggest that in breast cancer patients undergoing priming with rG-CSF, steady-state BM CD34+/CD71- measurement is a relevant predictive parameter of CD34+ mobilization.