The distribution of Chlamydia trachomatis serotypes in the sexually active population may change over time. Serum from C. trachomatis seropositive women representing the 1980s, 1990s, and 2000s were available from a stratified random sample (11,067) of the Finnish Maternity Cohort for microimmunofluorescence-based classification. The C. trachomatis serotype distributions in the 1980s and 2000s were comparable, with serotypes G, E, and J being the most prevalent. In the 1990s the numbers of women seropositive for ≥ 2 serotypes peaked, and serotypes G/J were replaced by serotypes E/D. The temporary C. trachomatis serotype replacement parallels changes in the sexually active population in the 1990s in Finland.