Type 1 15-hydroxyprostaglandin dehydrogenase (PGDH) is the main enzyme responsible for the metabolism of prostaglandin E2 (PGE2) and PGF2 alpha. To examine the possibility that a deficiency of PGDH might contribute to preterm labor, we measured localization of immunoreactive (IR-) PGDH, PGDH mRNA, and PGDH enzyme activity in chorio-decidua, placenta, and amnion in patients after term elective cesarean section (n = 9), after spontaneous vaginal term delivery (n = 10), and at idiopathic preterm labor (PTL) in the absence of infection (< 36 weeks gestation; n = 11). Localization of IR-PGDH was determined in additional specimens of membranes after PTL with infection (n = 13) and without (n = 37). IR-PGDH was localized in syncytiotrophoblast and intermediate trophoblasts in placenta and in the trophoblast layer of extraplacental chorion, but was absent from amnion in all patient groups. In chorion, the number of IR-positive trophoblasts was significantly reduced in the idiopathic PTL group compared to those in the other groups. The relative abundance of PGDH mRNA in the chorio-decidua, but not the placenta, from spontaneous labor and PTL was significantly less than that after cesarean section. PGDH mRNA in chorio-decidua from preterm patients correlated with PGDH enzyme activity. Undetectable or low IR-PGDH in chorionic trophoblasts was also associated with low enzyme activity. These results suggest that there exists a subset of patients that present in PTL because of reduced PGDH expression in chorionic trophoblasts. We suggest that this relative deficiency would allow PGs synthesized in the amnion or chorion to escape metabolism in the chorion and thereby contribute to the stimulus to idiopathic PTL.