Antibodies to herpes simplex virus type 2 (HSV-2) and Chlamydia trachomatis (Ct) were determined in sera from pregnant women from 1970 and at intervals up to 1993. The trends for HSV-2 and Ct infections were deduced from the observed antibody rates in different age groups during the observation period. Total antibody rates for HSV-2 tended to decline toward the end of the period when age-matched groups were compared, while the Ct antibody rates peaked in 1979 and then declined gradually. Age-specific antibody rates showed declining frequencies in women younger than 20 years for both HSV-2 and Ct over the study period. Women 35 years of age and older in the early 1990s had significantly higher antibody rates than younger women at that time or than women of similar age in the early 1970s. This group of slightly older women with high antibody rates in the 1990s were 15-20 years of age in 1970 when a high antibody frequency was noted in this age group. High antibody rates against both HSV-2 and Ct in older pregnant women in the early 1990s may thus reflect a high incidence of these infections around 1970. The declining rates of antibodies in the youngest women would suggest a declining incidence of primary infections in this group.