Aztreonam was administered to 145 consecutive patients with complicated urinary tract infections caused by multidrug-resistant, aztreonam-sensitive bacteria. Multidrug resistance was defined by disk diffusion testing as resistance to aminopenicillins and to first- and second-generation cephalosporins, with or without resistance to aminoglycosides. The first 40 assessable patients received 1 g of aztreonam intravenously three times daily for a median period of eight days; the remaining 95 assessable patients received 0.5 g twice daily for a median period of nine days. Fifty-five patients were infected with Pseudomonas aeruginosa, 24 with Escherichia coli, 18 with Serratia marcescens, 13 with Morganella morganii, 12 with Providencia rettgeri, and 10 with Enterobacter species. Bacteriologic cure rates were 98% for the group given 3 g daily and 96% for that given 1 g daily. Minimal and transient adverse reactions occurred with both dosages.