A multicentre prospective randomised open-label controlled trial was conducted to compare the efficacy and safety of aztreonam (1 to 2g every 8 hours) plus cefazolin (1g every 8 hours) with that of ceftazidime (1 to 2g every 8 hours) in patients with nosocomial pneumonia. 66 adults were enrolled who had a presumptive diagnosis of pneumonia made at least 48 hours after hospitalisation. Patients exhibited the classic clinical picture of pneumonia and/or fever and/or leucocytosis and a newly developed or expanding infiltrate on chest x-ray.Among 48 evaluable patients treated, clinical cure or improvement was reported in 18 of 20 (90%) aztreonam patients as compared with 24 of 28 (85.7%) patients treated with ceftazidime. Bacteriological eradication or presumptive eradication was reported in 11 of 13 (84.6%) aztreonam patients and 15 of 19 (78.9%) ceftazidime recipients. Five patients with positive blood cultures prior to therapy (2 aztreonam, 3 ceftazidime) had negative blood cultures at study completion.Superinfection was reported in 2 patients randomised to aztreonam (bothStaphylococcus aureus) and in 3 ceftazidime patients (2 withPseudomonas aeruginosaand 1 with &bgr;-haemolyticStreptococcus). Colonisation was observed following ceftazidime (n = 4) and aztreonam (n = 2) therapy. Six aztreonam and 3 ceftazidime recipients experienced adverse events, which were possibly related to study therapy.Aztreonam plus cefazolin appears to be as effective as ceftazidime for the treatment of adults with nosocomial pneumonia.