Multicenter trials were conducted to determine the safety and efficacy of aztreonam in the treatment of patients with gram-negative bacterial infections. A total of 2,821 patients were treated; 2,117 received aztreonam and 704 received control antibiotics. All patients were evaluated for safety and 1,180 of those treated with aztreonam and 428 treated with the control drugs met the criteria for efficacy evaluation. The number of patients treated with aztreonam who were evaluable for efficacy and their microbiologic response rates were: urinary tract infections, 443 (82 percent); lower respiratory tract infections, 217 (79 percent); septicemia, 63 (98 percent); skin/skin structure infections, 136 (88 percent); intra-abdominal infections, 47 (85 percent); postpartum/gynecologic infections, 21 (100 percent); bone and joint infections, 12 (100 percent); acute uncomplicated gonorrhea, 209 (97 percent); and acute uncomplicated cystitis, 56 (84 percent). Adverse reactions were qualitatively similar to those reported for beta-lactam antibiotics, i.e., mild gastrointestinal upset, rash, eosinophilia, or transient increase in hepatic enzyme parameters. There was an apparent lack of adverse effects on kidney, inner ear, and blood coagulation system. The most frequent adverse effect was phlebitis at infusion site (2.4 percent of patients). Superinfections and colonization with new organisms occurred in 9.4 percent of aztreonam-treated patients and in 7.4 percent of control drug-treated patients; only 40 percent of patients in each group, approximately 4 percent of all patients receiving aztreonam and 3 percent of those receiving control antibiotics, required specific therapy for the superinfection. Overall, results indicated that aztreonam is a safe and effective antibiotic in the treatment of aerobic gram-negative infections, when used either as monotherapy or in combination with other antibiotics.