During the second half of the 1980s, Miami had a syphilis epidemic while gonorrhea rates decreased. To determine whether the direction of these trends truly differed within all population subgroups or whether they resulted from aggregating groups within which trends were similar, records from four sexually transmitted disease clinics from 1986 to 1990 and census data from 1990 were used to compare race-, sex-, age-, and zip code-specific groups. Syphilis and gonorrhea clustering was similar; 50% of cases occurred in the same zip codes, representing 10% of the population. In all groups, gonorrhea decreased (aggregate 48%) while syphilis first increased (aggregate 47%) and then decreased. Determining reasons for these different trends may facilitate controlling these diseases.