Chorioamnionitis is one of the important causes of preterm labor. Preterm labor with chorioamnionitis is associated with oxidative stress. We reported that 4-hydroxy-2-nonenal (4-HNE), a major end product of oxidative fatty acid metabolism, is accumulated in the placenta with chorioamnionitis. The aim of this study was to confirm the effect of 4-HNE on cyclooxygenase-2 (COX-2) and prostaglandin (PG) induction in the uterine myometrial tissues. We also examined the effect of N-acetylcysteine (NAC) on 4-HNE-induced COX-2 expression. Uterine myometrial tissues were obtained from 5 patients. One of them underwent elective cesarean section without labor, and 4 of them underwent hysterectomy because of placental previa or atonic bleeding. We stimulated the uterine myometrial tissues with 4-HNE. In addition, the tissues were pretreated with NAC before 4-HNE treatment. The expression of COX-2 mRNA was observed by real-time PCR. PGE2 and prostacyclin release into the supernatants of the tissue cultures was measured by ELISA. 4-HNE induced the COX-2 mRNA expression and PGE2 production in the uterine myometrial tissue culture in a dose-dependent and time-dependent manner. NAC inhibited 4-HNE-induced COX-2 expression. 4-HNE may play an important role in preterm labor. NAC might be protective against preterm labor under oxidative stress. Copyright © 2011 S. Karger AG, Basel.