Summary— Interstitial cystitis comprises a complex of diseases typified by symptoms of pelvic pain. Functional complaints do not aid the clinician in determining loss of anatomical capacity. Histochemical staining with PAS-colloidal iron/Van Geison's counterstain offers improved diagnostic ability for the pathologist and correlates well with immunofluorescent findings. Four distinct diseases can be identified through immunofluorescent staining, indicating that each is the result of different responses of the urothelium and endothelium to injury. Loss of bladder capacity associated with these diseases can be expected with age, but immunofluorescent staining for IgM within the capillaries of the interstitium is a more sensitive predictor.