Aztreonam is a new, synthetic, monobactam beta-lactam antibiotic with activity directed specifically against aerobic gram-negative bacteria. At doses of 1-8 g per day, aztreonam was administered parenterally to patients with serious gram-negative infections of the urinary tract or other sites. A total of 462 organisms were isolated from sites of urinary tract infection; 93% of the isolates obtained within 48 hr before the initiation of aztreonam therapy were eradicated. Forty-three (86%) of 50 strains of Pseudomonas aeruginosa from urinary sites were eradicated. Eighty-six percent of gram-negative organisms isolated before treatment from other sites of infection were eradicated. Whatever the site of infection, the percentage of patients with a complete resolution of signs and symptoms was similar. Aztreonam was well tolerated both locally and systemically.