Bacterial colonization is the necessary first step in the etiology of all urinary tract infections. The hospital setting provides a pool of pathogenic bacteria which may lead to colonization in the hospitalized patient. The nosocomial infections which may follow and require treatment most often follow catheterization. Some host factors often lead to persistence of colonization and invasive symptomatic disease. These might include stone, caliceal diverticula, a hydronephrotic kidney, vesicoureteral reflux, bladder neck obstruction with residual urine, urethral diverticula, and infection of the prostate, as well as other disease states such as diabetes mellitus, cancer, or drug immunosuppression. Bacterial virulence must also be considered, but since normal host barriers are broken down by catheterization or instrumentation, usually avirulent bacteria are the more common cause of nosocomial urinary tract infections. Increasing bacterial resistance to antibiotics has been recognized among nosocomial infections. Thus, for a stringent trial, any new drug should be used to treat infections due to bacteria with multiple antibiotic resistance. The antibiotic must be active against the organisms, and early follow-up should evaluate both microbiologic and symptomatic cure. Late follow-up should show that there is no persistence of the original organism. New antibiotics should show some advantage over present antibiotics being used for treatment. Aztreonam offers an advantage over aminoglycosides in that it is neither ototoxic nor nephrotoxic. It appears to cause less antibiotic resistance in bacteria colonizing the intestine and therefore holds an advantage over most of the beta-lactam antibiotics. In addition, aztreonam does not appear to be haptogenic and has shown little cross-allergenicity to either penicillins or cephalosporins. Worldwide trials of the drug suggest that aztreonam is as efficient, if not more so, than the third-generation cephalosporins, newer penicillin derivatives, and the aminoglycosides in the treatment of Pseudomonas and other bacteria causing nosocomial urinary tract infections.