There is extensive evidence indicating that the capacity of Escherichia coli to attach to the mucosal lining of the urinary tract is a virulence factor in acute pyelonephritis in the unobstructed state. In vitro results using human uroepithelial cells and clinical E. coli isolates as well as in vivo work on ascending urinary tract infection in mice and E. coli strains with genetically defined adhesins support this notion. The biochemical characterization of the bacterial ligands and epithelial cell receptors important for the attachment of most pyelonephritogenic E. coli provides a more sophisticated means of evaluating the role of bacterial adhesion in urinary tract infection: 1) It allows precise diagnosis of the receptor specificity of clinical isolates; 2) The receptor can be used to isolate the relevant bacterial adhesins; 3) The localization and quantity of the receptor in the patient may be of prognostic importance; 4) The administration of soluble receptor analogues or antibodies to the adhesins may be useful for prophylactic and/or therapeutic purposes.