Sera from the mothers of all children from the greater Helsinki area who were treated in the Neonatal Intensive Care Unit (NICU) of Children's Hospital (University of Helsinki) during a 22-month period were studied serologically, and antibody levels for mothers were compared with those for matched controls. IgM to Chlamydia trachomatis serotype GFK was detected more often in sera from mothers with children in the NICU than in that from controls; IgM was detected in sera from 39 of 264 mothers vs. 15 of 274 controls (P < .001; two-tailed test). The gestational age of children born to mothers in the IgM-seropositive group was 32.4 weeks whereas that of children born to mothers in the IgG/IgM-seronegative group was 34.3 weeks (all children were treated in the NICU). The frequency of signs of maternal infection (i.e., fever and vaginal discharge), the frequency of meconium-stained amniotic fluid, the frequency of chorioamnionitis, and the mortality rate were higher in the IgM-seropositive group than in the IgM-seronegative group; pneumonia, atelectasis, and pneumothorax occurred more frequently in the seronegative group. Thus, in terms of birth weight and perinatal infections, the outcome was better for children whose mothers did not have antibodies to C. trachomatis.