Eosinophilia associated with tumors is extremely rare. The exact underlying pathogenetic mechanisms are not well understood; limited data have been reported about granulocyte colony-stimulating factor and interleukin-5 in the development of eosinophilia. We report a patient with marked eosinophilia, leukocytosis, and eosinophilic colitis presenting with bloody stool. The subsequent diagnosis was advanced high-grade transitional cell carcinoma of the bladder. Leukocyte count, eosinophil count, and colonoscopic findings returned to normal after surgical removal of the tumor. To our knowledge, eosinophilic colonic infiltration as a manifestation of transitional cell bladder carcinoma has not been previously reported.