There are several areas in which the use of aztreonam seems logical. Infections caused by organisms sensitive to aztreonam that are known to be multiresistant to other agents can be treated directly with aztreonam in single, directed therapy, thus making the use of more toxic agents unnecessary. In types of infection in which both gram positive and gram negative bacteria are present, aztreonam can replace the usual aminoglycoside component of the therapeutic regimen. In settings of mixed infections suspected of being caused by drug-resistant strains of Enterobacteriaceae and/or P. aeruginosa, aztreonam can be combined with an agent active against gram positive organisms or with one active against anaerobes. Aztreonam has proven to be effective, safe therapy for serious and life-threatening infections caused by multiresistant aerobic gram negative bacteria. It should be used in combination with drugs that inhibit gram positive species if the etiology of the infection is not known, particularly in the immunocompromised, neutropenic patient. Doses of 1 g every 8 to 12 hours will be adequate for treatment of infections caused by most Enterobacteriaceae. Whether 2 g doses every 8 hours would be preferred for treatment of systemic Pseudomonas infections remains to be determined. Urinary infections caused by gram negative bacteria can be treated with 500 mg administered IM once or twice daily. The dosage of aztreonam should be adjusted in patients with renal failure. Clearly, aztreonam is a useful addition to the antimicrobial agents available to the physician.