Cytapheresis with extra-corporeal circulation for ulcerative colitis is effective but its mechanisms are still unclear. Granulocytecolony stimulating factor (G-CSF) strongly mobilizes bone marrow-derived cells and serves as antiinflammatory factor. We investigated plasma levels of G-CSF during granulocyte and monocyte adsorptive apheresis (GCAP). Nineteen cases of ulcerative colitis were measured plasma concentration of G-CSF during the first session of GCAP therapy. G-CSF were significantly increased in the column inflow at 30 min compared with the baseline (Wilcoxon test, p < 0.01), and also increased through the column (Wilcoxon test, p < 0.01). The ratio of the increase in the column outflow at 60 min was 1.5-fold compared with the baseline. However, we could not show a significant relation between G-CSF level and clinical efficacy. This is the first report concerning G-CSF during CAP. G-CSF is increased due to GCAP and appears to be a candidate which should be further investigated.