Antimicrobial susceptibilities of Neisseria gonorrhoeae have been prospectively determined in the Gonococcal Isolate Surveillance Project of the Centers for Disease Control and Prevention. Form 1988 through 1994, susceptibities were determined for 35,263 isolates from 27 clinics. Patients were demographically similar to the in nationally reported gonorrhea cases. In 1994, 30.5% of isolates had chromosomally or plasmid-mediated resistance to penicillin or tetracycline. Penicillin resistance increased from 1988 (8.4%) to 1991 (19.5%) and then decreased in 1994 (15.6%). Tetracycline resistance decreased from 1988 (23.4%) to 1989 (17.3%) and then increased in 1994 (21.7%). Most isolates (99.9%) were highly susceptible to broad-spectrum cephalosporins. Isolates with decreased susceptibility to ciprofloxacin increased susceptibility to ciprofloxacin from 1991 (0.4%) to 1994 (1.3%); 4 isolated were ciprofloxocin-resistant. Ciprofloxacin-resistant strains may not respond to therapy with recommended doses of flouroquinolones, and the clinical importance of stains with decreased susceptibility is unknown. The emergence of fluoroquinolone resistance in N. genorrhoege in the United States threatens the future utility of this class of antimicrobials for gonorrhea therapy.