Aztreonam--a new, synthetic, monocyclic beta-lactam antibiotic with excellent in vitro activity and beta-lactamase stability--was used for the treatment of 26 serious infections due to gram-negative bacteria in 23 patients: nine cases of bacteremia, one of endocarditis, one of pneumonia, one of septic arthritis, six of osteomyelitis, five of abscess or soft tissue infection, and three of meningitis. The majority of patients had serious underlying disease, and 18 were in critical or poor condition. The mean age of the patients was 62 years, and the mean duration of therapy was 19 days. The clinical condition of all 23 patients improved during therapy; 20 infections were cured according to clinical criteria. Three of the six instances of therapy failure were due to inadequate debridement. No superinfections, resistant pathogens, or significant adverse reactions were seen. Aztreonam was effective and safe for the treatment of serious gram-negative infections.