Serological examination by complement fixation (CFT) with group-specific and ornithosis species antigen was carried out on 723 blood serum specimens from women with obstetrical pathology, 124 specimens from patients with various chronic gynecological diseases and 124 control blood serum specimens from women with normal pregnancy and favorable obstetrical anamnesis in order to determine the role of chlamydial infection in pathological pregnancies. Complement-fixing (CF) antibody was detected in 72 (9.9%) serum specimens from women with obstetrical pathology and chronic gynecological diseases. No CF antibody was found in control sera. Negative results of the CFT were obtained with ornithosis antigen in all sera. Parallel examinations of the serum from women with obstetrical and gynecological pathology by CFT and intradermal test showed correlation in 62.5% of the patients. Among the women with positive reactions for chlamydial infection, in 52.8% the pregnancy was pathological. Among 52 seropositive pregnant women, fetal death occurred in the perinatal period in 21.2%. Among 111 babies born alive after previous pregnancies, 55 babies (49.5%) died under 1 yr of age. Thus, chlamydial infection may have a causative relation to pathological pregnancy, death of the fetus in the perinatal period and death of babies under 1 yr.