The normal bladder is highly resistant to infection. It also appears that those infections which do develop in the properly functioning healthy bladder quickly disappear unless maintained by kidney lesions. A notable exception to these observations is found in the frequent occurrence of cystitis following pelvic operations. In search for an explanation of this complication, we discovered, eight years ago, that patients who are repeatedly catheterized after operation usually fail to regain immediately the power of complete evacuation. This discovery led to the suspicion that residual urine may be an important factor in the development of postoperative cystitis. A second exception to the rule that the bladder is resistant to infection and tends to heal quickly is found in the occurrence of pyelitis during the course of pregnancy and in the puerperium. And here, again, I found that retention of several ounces of residual urine is frequently demonstrable. If a