Traditional concepts of impermeability of the bladder have centered around unique cellular tight junctions and ion pumps. However, recent data from our laboratory have shown that the bladder epithelium in animals and humans relies primarily on its surface glycosaminoglycans to maintain its impermeability. This study demonstrates the first disease associated with an epithelial dysfunction of the bladder, that is a leaky epithelium. The study consisted of 31 normal subjects and 56 individuals with interstitial cystitis. Interstitial cystitis patients were shown to have a leaky epithelium by placing a solution of concentrated urea into the bladder and measuring the absorption. The normal subjects absorbed 4.3% in 45 minutes, while the interstitial cystitis patients absorbed 25% (difference is highly significant, p less than 0.005). Interstitial cystitis patients with Hunner's ulcers (10) had a 34.5% absorption rate, while those without ulcers absorbed 22.8% (46). This difference also was highly significant (p = 0.002) and supports the concept that patients with ulcers have clinically worse disease.