Aztreonam and cefotaxime were compared in 44 cirrhotic patients who had 52 episodes of gram-negative spontaneous peritonitis. Patients were randomized into two therapeutic groups of similar characteristics. Group A (28 episodes) received 0.5 gm of aztreonam every 8 hr, and group B (24 episodes) received 1 gm of cefotaxime every 6 hr, for a planned 14-day period. Peak and trough serum and ascitic fluid levels of both antibiotics were several times higher than the minimum inhibitory concentrations of causative microorganisms. Eleven patients (21%) died within the first 48 hr after beginning therapy, which included seven in the aztreonam group and four in the cefotaxime group. In the remaining patients, signs and symptoms of infection were promptly controlled, and ascitic fluid cultures became negative after 48 hr in all cases, except in one patient from the aztreonam group, who was a clinical failure. Two patients from the aztreonam group and one from the cefotaxime group relapsed after treatment. The overall mortality rate was 50%, which was lower than classically reported: 12 patients (43%) died in the aztreonam group, and 14 (58%) died in the cefotaxime group (p = 0.265, NS). Hepatorenal syndrome and digestive tract hemorrhage were the most frequent causes of death occurring after the first 48 hr of treatment. Streptococcal superinfections developed in three patients (14.2%) in the aztreonam group. We conclude that both antibiotics at the low doses used in this study are similarly well tolerated and effective in controlling this infection. Because the use of aztreonam as the initial empirical treatment requires a concomitant antibiotic against gram-positive infections and the possibility of streptococcal superinfections, cefotaxime seems to be a more advantageous therapeutic alternative for this patient population.