BACKGROUND:Rates of fluoroquinolone-resistant Neisseria gonorrhoeae (QRNG) are increasing worldwide and in California.METHODS:As a supplement to established surveillance, the investigation of QRNG in California included expanded surveillance in southern California, with in-depth interviews of patients (who had QRNG during the period of January 2001-June 2002) and a cross-sectional study of patients at 4 sexually transmitted diseases clinics with gonococcal isolates that underwent susceptibility testing (for the period of July 2001-June 2002).RESULTS:The rate of QRNG increased from <1% in 1999 to 20.2% in the second half of 2003. The 2001-2002 expanded surveillance demonstrated that 66 (4.9%) of 1355 isolates were resistant to fluoroquinolones; the majority of these infections occurred after August 2001. Cross-sectional analysis of 952 patients with gonorrhea revealed that the prevalence of QRNG varied geographically during 2001-2002, with the highest rate being in southern California (8.9%) and the lowest being in San Francisco (3.6%). The QRNG prevalence was 8.6% among men who have sex with men (MSM), 5.1% among heterosexual men, and 4.3% among women. Although risk factors for QRNG varied by clinic, multivariate analysis demonstrated independent associations with race/ethnicity, recent antibiotic use, and MSM.CONCLUSIONS:The emergence and spread of QRNG in California appeared to evolve from sporadic importation to endemic transmission among both MSM and heterosexuals. Monitoring of both the prevalence of and risk factors for QRNG infections is critical for making treatment recommendations and for developing interventions to interrupt transmission.