A serological test for chlamydial infection was administered to 281 Jerusalem women in order to determine the rate and influence of Chlamydia on pregnancy outcome. Serological indication of active infection was present in 7.8% of the tested women, while 15.3% were shown to be positive for Chlamydia. Among the ultraorthodox subpopulation of Mea Shearim, serological indication of active infection was present among 5.9% of the women, and 12.3% of this population tested positive. In comparison, women from the secular subpopulation had 12.7% serological indication of active infection and 22.95% tested positive (P < 0.01). There were no statistically significant differences between pregnancy duration, birthweight, incidence of premature uterine contractions, premature rupture of membranes, and postpartum febrile morbidity in the infected and noninfected groups. Women with a previous history of induced abortions showed a significantly higher evidence of past Chlamydia infection (9.3%) when compared with the women who did not have an infection (1.4%) (P < 0.006). Among the ultraorthodox women with positive or active infection, 41% had suffered at least one spontaneous abortion, as compared with 25% of the religious women who had no serological evidence of infection.