Leukocytapheresis is effective for ulcerative colitis (UC), and both peripheral leukocyte and platelet counts decrease during apheresis. While leukocyte counts increase and overshoot after finishing the apheresis procedure, platelet reduction continues even after apheresis. The aim of this study is to elucidate the impact of the overshoot of leukocyte and continuous platelet reduction on the efficacy of leukocytapheresis for UC. Leukocyte and platelet counts before and one hour after each leukocytapheresis session of 43 UC cases were examined (31 entered remission, but 12 did not). The correlation between the efficacy of leukocytapheresis and leukocyte overshoot or platelet reduction was examined. The average increase ratio of leukocytes in the initial five sessions of the responders was not significantly different from that of the non-responders (95% vs. 92%, P = 0.28). In contrast, the average decrease ratio of platelets in the initial five sessions of the responders was significantly higher than that of the non-responders (34% vs. 27%, P = 0.0041). Moreover, the platelet reduction at the third and fourth apheresis sessions in the responders was remarkably greater than that in the non-responders (33% vs. 20%, P = 0.0018). The efficiency of platelet reduction could be a predictive factor for the efficacy of leukocytapheresis.