To investigate the role of granulocytes and the effect of granulocytosis on malignancies, we studied the relationship between white blood cell and granulocyte (Gr) counts, and tumor size. As we have reported that cancer patients with high granulocyte (G/L) ratio have a poor clinical prognosis, normalization of the G/L ratio could contribute to improvement in their condition. Tumor bearing rabbits treated with the extracorporeal granulocytte depletion device showed a decrease in tumor size and no tumor necrosis. Tumor-bearing rabbits with no extracorporeal granulocyte depletion showed a remarkable tumor progression and an increase in Gr count. Depletion of granulocytes considerably improved the survival ratio of tumor-bearing animals. The device used in this study could be a potential treatment for malignant tumors.