Of 146 hospitalized patients treated with aztreonam for infections, primarily of the urinary and lower respiratory tracts, 142 had a satisfactory clinical response. An initial microbiologic cure was found in 106 (84%) of 126 patients. Relapse, reinfection, or superinfection occurred in 28 patients. Thirty-nine of 43 patients treated with a control drug (gentamicin, cefotaxime, cefamandole, or tobramycin) had a satisfactory clinical response, 25 of 34 such patients showed microbiologic cure, and 12 experienced relapse, reinfection, or superinfection. Short-term perioperative treatment with aztreonam in male patients whose urine was positive for gram-negative organisms was successful for 10 of 11 patients. Pharmacokinetic studies demonstrated that dosage adjustment of aztreonam in geriatric patients should be based on creatinine clearance. No nephrotoxicity was demonstrated in 10 healthy male volunteers. Satisfactory concentrations of aztreonam were found in sputum, vagina, cervix, perimetrium, myometrium, endometrium, parametrium, fallopian tube, fat, muscle, and kidney tissues.