Kits for measurement of C. trachomatis specific antibody are not commercially available, but of the significance of determination of the serum antibody level has not been clarified. In this study, we first determined the C. trachomatis antibody level in patients considered to be free of C. trachomatis infection. Then, we evaluated the relationship between the presence or absence of antigen with the antibody level in patients at our department who were available for antigen examination. Anti-C. trachomatis IgA and IgG antibodies were detected in all age groups with a peak frequency in the 20's expect that IgA antibody was absent in the umbilical blood (IgG antibody was detected also in the umbilical blood). Concerning the relationship between the presence or absence of C. trachomatis antigen and the serum antibody levels, both IgA and IgG antibodies were significantly different (chi 2 test, p less than 0.01) between the antigen-positive group and the antigen-negative group. Because females infected by C. trachomatis were more often asymptomatic than infected males, and because antigen tests may produce false negative results, determination of the antibody level must be included in the clinical tests. Determination of the antibody level is considered to be valuable also in screening for mother-child infection and asymptomatic infection.