PROBLEM/CONDITION: From 1986 through 1990, an epidemic of syphilis occurred throughout the United States. In 1991, the number of reported cases of primary and secondary (P&S) syphilis in the United States declined for the first time since 1985. REPORTING PERIOD COVERED: To examine how this decline reflected sex-specific, race/ethnicity-specific, and regional patterns of syphilis morbidity, we analyzed data for syphilis cases reported to CDC from 1984 through 1991. DESCRIPTION OF SYSTEM: Summary data for cases of syphilis reported to state health departments were sent quarterly and annually to CDC. The quarterly data from each state included total number of syphilis cases by sex, stage of disease (primary, secondary, early latent, and late latent), and source of report (public or private). The annual data from each state included total number of P&S syphilis cases by sex, racial/ethnic group (white, not of Hispanic origin; black, not of Hispanic origin; Hispanic; Asian/Pacific Islander; or American Indian/Alaskan Native), 5-year age group, and source of report. RESULTS: The decline in both the number and rate of reported syphilis cases in 1991 occurred in every racial group in the United States and in both sexes. This decline also occurred in every region of the United States except the Midwest, where the total P&S syphilis rate increased 37.3% from 1990 through 1991. Despite the increase in syphilis rates in the Midwest, the highest rates of P&S syphilis in 1991 were reported from the South. INTERPRETATION: The reasons for the decline in syphilis are unclear. No data exist to conclusively identify which STD control program activities affected the level of syphilis morbidity or to what extent those activities may have contributed to the decline. Changes in drug use and limited immunity to Treponema pallidum may have accounted for some of the decrease in syphilis incidence. Higher levels of poverty in the South and poor access to health-care services associated with poverty probably contributed to continued high levels of disease transmission in the South. ACTIONS TAKEN: Better evaluation of STD control program activities will be necessary to help determine the most effective strategies for preventing and controlling syphilis in different high-risk populations.