In a prospective, randomized trial, aztreonam (1 g intravenously or intramuscularly, twice daily) was compared with ceftazidime (1 g intravenously or intramuscularly, twice daily) and amikacin (500 mg intravenously or intramuscularly, twice daily) in 76 patients aged 24 to 84 years (mean, 59.7 years) with complicated urinary tract infections. Initial pathogens included Escherichia coli (47.5%), Pseudomonas aeruginosa (22.5%), Klebsiella spp. (9%), Proteus spp. (7.5%) and Enterobacter spp (6%). In four patients initial urine cultures yielded more than one organism. All pathogens were sensitive to the three study drugs. Including performance of 4- to 6-week follow-up cultures, eradication of the pathogens occurred in 72% of patients treated with aztreonam, in 74% of those treated with ceftazidime and in 71% treated with amikacin (p > 0.05). Clinical success was observed in 84% of patients treated with aztreonam, in 82% of those treated with ceftazidime and in 85 % treated with amikacin (p > 0. 05). All drugs were well tolerated. It is concluded that aztreonam, ceftazidime and amikacin are equally effective and safe for the treatment of complicated urinary tract infections due to susceptible organisms.