Placental ALP (ALP(h)) is a membrane-anchored, heat-stable enzyme produced by the syncytiotrophoblast. We report a case of a patient presenting in the third trimester with extreme increased levels of ALP(h). A 40-year-old woman, gravida 2, para 1, was admitted to the high risk pregnancy unit at 30 weeks of gestation for evaluation of an incidental finding of marked isolated elevation in serum ALP(h). Blood tests obtained at admission revealed a marked elevation of serum ALP level up to 1,194 u/l (reference 35-104 u/l). At 36 weeks of gestation, the patient was admitted with preterm premature rupture of membranes. Due to breech presentation, a cesarean delivery was performed. An immunoperoxidase stain was done for placental alkaline phosphatase, which was positive in the majority of chorionic trophoblastic cells. No evidence of inflammation was detected in the placental chorionic plate. Thus, isolated elevation in placental ALP may be linked to a subsequent preterm delivery.