A case of shock following intravesical formalin instillation for massive bladder hemorrhage owing to post-radiation cystitis is reported. A 79-year-old woman had been treated with external irradiation for cancer of the uterine body in 1981. She was suffering from massive hematuria after hysterectomy in 1988. No hemostatic procedures were effective. Then we instilled 10% formalin into her bladder. After the instillation she entered a state of allergic shock.