Leakage of tight junctions as observed with electron microscopy and demonstration of solute transport across bladder epithelium was investigated in 13 patients with different bladder diseases: urinary retention and infection, bladder tumours and interstitial cystitis. The latter group showed consistent leakage of solutes through the urothelium. Electron microscopy revealed leaky tight junctions. This finding might be helpful in establishing the histological criteria and explain the pathogenesis of interstitial cystitis.