In a multicenter open study, intraperitoneal aztreonam was used together with vancomycin, cloxacillin, or flucloxacillin for initial empiric treatment of peritonitis associated with continuous ambulatory peritoneal dialysis (CAPD). Monotherapy with aztreonam was continued in 34 episodes of gram-negative peritonitis in 28 patients. The microorganisms isolated included Escherichia coli, Acinetobacter species, Pseudomonas species, and Klebsiella species. In three episodes, two organisms were cultured. Microbiologic assessment revealed cure in 27 episodes, cure with relapse in two, cure with superinfection in one, and treatment failure in four (with resistance to aztreonam in three). As assessed by clinical criteria, 27 episodes were cured, five failed to respond, and two responded partially. No adverse reactions to aztreonam were observed. Comparison of these 34 episodes with 35 episodes in historical controls (treated mainly with aminoglycosides and/or trimethoprim-sulfamethoxazole) showed significantly higher rates of cure (84% vs. 51%) and of survival (97% vs. 86%) as well as lower rates of catheter removal (65% vs. 87%) with aztreonam. Thus aztreonam is a safe and efficient agent for the treatment of peritonitis caused by gram-negative organisms in patients undergoing CAPD.