BACKGROUND: The short-term effects of granulocyte-colony-stimulating factor (G-CSF) have been extensively studied, but recent reports of G-CSF-induced genetic perturbations raised concerns regarding its long-term safety. In this respect, duration of G-CSF-induced perturbations has been less studied than short-term effects and needs to be evaluated. STUDY DESIGN AND METHODS: G-CSF mobilization-induced immunologic alterations were prospectively analyzed in a cohort of 24 healthy donors. Blood samples were taken before G-CSF administration; at the time of administration; and at 1, 3, 6, and 12 months and analyzed for blood cell counts and in vitro cytokines (interleukin [IL]-2, -8, and -10) and immunoglobulin production, quantified in the culture supernatant of peripheral blood mononuclear cells (PBMNCs) after, respectively, phytohemagglutinin and pokeweed mitogen stimulation. RESULTS: Platelet, granulocyte, monocyte, B, and dendritic blood cell counts as well as the IL-2, -8, and -10 secretion by PBMNCs, perturbed at the time of G-CSF mobilization, returned to baseline values at 1 month, with T-cell and natural killer cell counts recovering at 3 months. In vitro immunoglobulin production was increased up to 6 months after mobilization. CONCLUSION: Although assessment of the potential long-term risk of G-CSF administration will require prolonged observation of larger cohorts, our data show that the duration of immunologic perturbations may be more persistent than previously anticipated, especially for B-cell functional alterations. Most perturbations remain, however, transient with a return to baseline values within 1 year.