Cefazolin was used to treat 146 men and women with uncomplicated gonorrhea. Among patients reexamined within two weeks, treatment failures occurred in 42% of 31 men treated im with 1 g of cefazolin and in 10% of 76 men and 24 women given 2 g im, Failure rates increased progressively with increasing in vitro resistance to cefazolin of isolates of Neisseria gonorrhoeae obtained before treatment. Cefazolin compares favorably with other cephalosporins in the treatment of gonorrhea, but in the doses evaluated, was not as effective as the antibiotic regimens currently recommended by the U.S. Public Health Service. However, concentrations of cefazolin in serum through 12 hr after administration of a 2-g im dose were increased markedly with simultaneous administration of probenecid, and the use of cefazolin with probenecid warrants further evaluation in the treatment of gonorrhea.