The trophoblast cells are in direct contact with endometrial tissues throughout gestation, playing important early roles in implantation and placentation. The physiologic significance and the operating mechanisms involved in probable altered trophoblast functions following Chlamydia trachomatis infection were investigated to determine if C. trachomatis initiates productive infection in trophoblast, effects of such event on the biosynthesis of cholesterol and its derivatives estrogen and progesterone; and the regulator of the biosynthesis of these hormones, human chorionic gonadotropin. Chlamydia trachomatis exhibited productive infection in trophoblast typified by inclusion formation observed when chlamydia elementary bodies were harvested from trophoblast and titrated onto HEp-2 cells. Assessment of the status of C. trachomatis in trophoblast showed a relative increase in protein of HSP-60 compared with MOMP, features suggestive of chlamydial chronicity. There was a decrease in cellular cholesterol of chlamydia infected trophoblast and a down regulation of HMG-CoA reductase. The levels of estrogen and progesterone were decreased, while the expression of aromatase and adrenodoxin reductase was up regulated. Also, there was a decrease in human chorionic gonadotropin expression. The implications of these findings are that C. trachomatis infection of trophoblast may compromise cellular cholesterol biosynthesis, thus depleting the substrate pool for estrogen and progesterone synthesis. This defect may impair trophoblast functions of implantation and placentation, and consequently affect pregnancy sequelae.