We continued the epidemiological survey on sexually transmitted disease due to Chlamydia trachomatis in Okayama Prefecture. The results obtained from October 1984 to October 1988 are briefly summarized in this paper. 1) Sera from outpatients with suspected chlamydial infections were tested for IgG antibodies to Chlamydia trachomatis by the microplate immunofluorescence antibody technique (MFA). Fifty five percent of the male and 61% of the female patients were positive for anti C. trachomatis IgG. 2) Clinical swabs obtained from 735 males and 251 females were examined for Chlamydia trachomatis antigens by the isolation method and direct immunofluorescence tests (DFA) were carried out simultaneously. C. trachomatis was recovered from 34% of the male and 22% of the female patients. 3) The age structure of the female patients was younger than the male patients. Female patients under 29 years of age were 57% of all female antigen-positive cases.