Twenty-one patients with serious gram-negative infections were treated with aztreonam. Twenty of these were clinical and microbiologic cures; there was one clinical improvement with microbiologic persistence. No bacteria became resistant. Cure rates were: bone and joint (11 of 11); skin and soft tissue (six of six); pneumonia (two of two); perinephric abscess (one of one); and intra-abdominal abscess (zero of one). The bacteria responsible for these infections included Pseudomonas aeruginosa (12), Serratia marcescens (two), Enterobacter gergoviae (three), Enterobacter aerogenes (two), Escherichia coli (one), Citrobacter diversus (one), and Hemophilus influenzae (one). Aztreonam was well tolerated. Significant serum glutamic-oxaloacetic transaminase/serum glutamic-pyruvic transaminase elevations developed in three patients, but none was symptomatic and all resolved after therapy was stopped. Two patients in whom a rash developed were receiving other antibiotics (vancomycin and metronidazole), making the cause of the rash unclear. Diarrhea developed in a single patient with Pseudomonas osteomyelitis, who also was receiving cefazolin for Staphylococcus aureus superinfection of his decubitus ulcer. Aztreonam was highly effective against gram-negative bacilli, including P. aeruginosa. The only clear-cut side effect was an asymptomatic rise in serum glutamic-oxaloacetic transaminase/serum glutamic-pyruvic transaminase levels in three patients.