PROBLEM/CONDITION: Chlamydia is the most common sexually transmitted bacterial pathogen in the United States; however, no precise data on the prevalence and incidence of chlamydia infection are available because currently no comprehensive national surveillance system exists for chlamydia. Despite the absence of such a system, states do report numbers of male and female chlamydia cases to CDC on a quarterly basis. REPORTING PERIOD COVERED: This report summarizes and reviews the chlamydia surveillance data received by CDC from 1987 through 1991. DESCRIPTION OF SYSTEM: Summary data on cases of chlamydia reported to state health departments were sent quarterly to CDC in Atlanta, Georgia. The quarterly data from each state included total number of chlamydia cases by sex and by source of report (public, private). RESULTS: From 1987 through 1991, the number of states with legislation mandating reporting of chlamydia increased twofold. The reported chlamydia rate from those states also doubled during the same time period, from 91.4 cases per 100,000 population in 1987 to 197.5 cases per 100,000 population in 1991. INTERPRETATION: This twofold increase in the rate of chlamydia reported to CDC did not represent a doubling in chlamydia prevalence or incidence during this time period. Instead, the increase resulted from the increase in the number of states with reporting laws and from the initial attempts of those states to identify and report diagnosed chlamydia infections. ACTIONS TAKEN: More accurate measures of the number of chlamydia infections and of trends in the chlamydia infection rate are needed to justify, develop, and evaluate public health programs to control chlamydia infections. An outline of possible surveillance activities for local communities is presented.