Summary—The morphological response of the bladder surface to acute and chronic urinary tract infection (UTI) was investigated using both scanning and transmission electron microscopy of bladder biopsies taken during routine cystoscopy. Acute bacterial infection only minimally altered bladder epithelium, whereas in chronic infections the surface layers showed gross changes which included loss of the outer layers of epithelial cells and alteration of the bladder contour. In addition, more erythrocytes and polymorphonucleocytes were present in chronic infection. In both acute and chronic urinary infection, bacteria were confined to a limited number of sites on the bladder surface and appeared to be arranged in microcolonies, but more bacteria were attached to bladder surfaces in chronic than in acute UTI. These findings were confirmed by studying epithelial cells shed from patients with proven urinary infections.A few patients who had been treated with antibiotics before cystoscopy and biopsy were also studied. These antibiotics included cephalosporins which had been previously shown to produce bacterial morphological variationsin vitro.Some of these changes were subsequently observed in the biopsies and the results suggest mechanisms whereby bacteria involved in urinary tract infection can survive exposure to antibiotic