Therapeutic results are given for 342 patients treated with various doses of aztreonam in 20 studies. One hundred sixty-five patients had urinary tract infections; 40 of these patients were bacteremic. Favorable clinical response was reported for 91.5% of the patients, and only one bacteremic patient failed to respond to treatment. Pneumonia was treated in 99 patients, 11 of whom were bacteremic; 86 responded favorably to treatment. In 22 patients with bacteremia of unknown focus, only one failure was noted. Of 368 bacterial isolates, 323 were eliminated during treatment. Pseudomonas aeruginosa was eradicated in 28 of 41 infections. Adverse reactions were rare and led to discontinuation of aztreonam treatment in only one patient. In 37 patients superinfections with 42 bacterial strains were reported. Four of these patients, all of whom were seriously debilitated by underlying conditions, had staphylococcal septicemia and died. One patient developed bacteroides septicemia but recovered. In seven patients with pneumonia, superinfections with Streptococcus pneumoniae were reported. The use of aztreonam in combination with antibiotics active against gram-positive and/or anaerobic organisms should be considered in treatment of infections other than those of the urinary tract.