In 1985, 2,455 cases of infection due to penicillinase-producing Neisseria gonorrhoeae were reported from Dade County, Florida. These cases represent the largest concentration of PPNG infections reported to date in the United States. A zip code-based geographic analysis of morbidity caused by gonorrhea and PPNG identified a core area where the percentage of all gonorrhea cases due to PPNG was 31-36%. This percentage decreased in a radial pattern outward from the central core. A prospective study of patients presenting to the Dade County central clinic with gonorrhea found that men infected with PPNG were more likely to have medicated themselves with illicit antibiotics than were patients infected with penicillin-sensitive N. gonorrhoeae (odds ratio, 3.6; 95% confidence interval, 1.9-6.8). Of all self-medicated patients, 42% obtained antibiotics via illicit over-the-counter purchase at pharmacies in the hyperendemic area. In contrast to previous reports, PPNG infection was not associated with either narcotic- or prostitution-related activities. The results show that PPNG is endemic in Dade County and is arrayed in a core distribution. Antibiotic abuse may have played an important role in the Miami epidemic and in the establishment of PPNG as an endemic strain.