We examined the prevalence of chlamydial infection in a population of pregnant women and observed their infants to determine the risk of development of ocular or respiratory infection. We examined endocervical and serum specimens from 322 pregnant women for Chlamydia trachomatis and chlamydial antibody. The cultures were obtained at the first prenatal visit. Six (2%) of the women were infected with C trachomatis. Chlamydial antibody was present in the genital secretions of 47% and 73% of the serum samples. The six infants born to infected women, 61 infants born to women who were culture-negative, but local antibody-positive, and 28 control infants born to culture-negative, antibody-negative women were followed for up to six months. Four of six infants born to infected women developed chlamydial infection: two developed culture-positive conjunctivitis, one had asymptomatic nasopharyngeal infection, and one infant developed pneumonitis. Three of 61 infants born to mothers who were culture-negative and local antibody-positive developed conjunctivitis due to C trachomatis. None of the 28 control infants developed chlamydial infection. Most (79%) of the infants had chamydial antibody in their serum at 2 to 4 weeks of age. The correlation between maternal and infant serum antibody titer was r=0.71 suggesting that antibody was placentally transferred.