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The short-term immunological effects of granulocyte-colony stimulating factor (G-CSF) administration have been extensively studied, but concerns regarding its long-term safety have been raised by recent reports such as G-CSF-induced genetic perturbations (Nagler et al, Exp Hematol, 2004) as analyzed in a limited number of donors. In response to such concerns, we conducted a prospective study to further evaluate both short and long-term immunologic and cytogenetic alterations potentially induced by G-CSF mobilization in a cohort of 24 consecutive healthy donors. Blood samples were taken before and at time of G-CSF administration and of cytapheresis as well as 1, 3, 6 and 12 months after. G-CSF treatment followed by cytaphereris induced a significant blood platelet count decrease while granulocyte, monocyte, B as well as dendritic cell (monocyte-derived DC and even more so plasmocytoid DC) blood counts increased. Such perturbations were found to return to baseline values at one month. T-cell (both CD4 and CD8) as well as NK-cell counts decreased significantly after cytapheresis and did not recover normal values before 3 months. In vitro immunoglobulin G and M production by pokeweed mitogen-stimulated PBMC was found to be significantly (P < 10−5) increased of 63+/−7% and 84+/−7% over baseline values; increases that persisted up to 6 months after mobilization. Such an observation is in line with our previous findings pertaining to enhanced humoral immune reconstitution after allo- PBSCT (Lapierre et al, Blood 2001, Blood 2002; Tayebi et al., Br J Haematol 2001). Quite strikingly, G-CSF mobilization was also associated with significant (P < 10−6) increased PBMC aneuploidy, analyzed by in situ hybridization of chromosomes 8 and 17 centromere-specific fluorescent probes. Such increase in the frequency of aneuploïd cells was not observed in CD34+ selected cells, suggesting that G-CSF mobilization did not affect CD34+ cells with regard to cytogenetic abnormalities and that the increase in aneuploidy was induced in more mature cells. The frequency of chromosomes 8 and 17 aneuploidy increased an average of 4.5 and 4.3-fold, respectively, after G-CSF mobilization (before mobilization: 1.2+/−0.1% and 2.4+/−0.2%; after mobilization: 4.2+/− 0.4% and 8.8+/−0.4% for chromosome 8 and 17, respectively). Cells with multisomy, rare events before mobilization (recorded in 5/24 donors and at very low levels (< 0.2%)), were observed at significantly higher frequencies in all 24 donors after mobilization. On the other hand, cells with monosomy and cells with trisomy were detectable at baseline in all donors although at low frequencies which increased significantly after mobilization. The level of aneuploidy subsequently decreased after G-CSF mobilization, but, importantly remained significantly higher than baseline values at three and six months after mobilization. The frequency of cells with monosomy was higher, and returned to baseline values more gradually than the frequency of cells with trisomy or multisomy. In conclusion, our data show that immunological and cytogenetic perturbations induced by G-CSF administration in normal donors may be more pronounced and persistent than previously described. Such perturbations remain however transient with a return to baseline values within one year for most perturbations. Consequences, if any, of such alterations for the donor are presently unknown. Although EBMT and NMDP surveys have no shown any evidence in favor of an increased incidence of malignant hemopathies or solid tumors in mobilized normal donors, assessment of the potential long-term risk of G-CSF administration will require prolonged observation of large G-CSF-mobilized donor cohorts.