Transcription factors orchestrate the development of extraembryonic tissues. Because placental hypoxia likely plays an important role in both normal and abnormal placentation, we have been investigating the hypoxia-inducible transcription factors (HIFs) in the human placenta. In this report, we focus on the placentas from women with preeclampsia. Because the placenta is a large, heterogeneous organ, we employed a systematic and unbiased approach to placental sampling, and our results are based on the analyses of eight biopsy sites per placenta. We observed no significant differences in HIF-1α or -2α mRNA expression between normal term and preeclamptic placentas. Nor was HIF protein expression significantly different, with the notable exception of HIF-2α, which, on average, was increased by 1.7-fold in the preeclamptic placentas (P < 0.03 vs. normal term placentas). Considering all 48 paired placental biopsy sites (eight sites each for six normal term and six preeclamptic placentas), HIF-2α protein levels in the preeclamptic placentas exceeded those in the normal term placentas in 39, or 81%, of the paired sites (P < 0.0013). The HIF-2α immunoreactivity was mainly located in the nuclei of the syncytiotrophoblast and fetoplacental vascular endothelium in the preeclamptic villous placenta. To control for the earlier gestational age of the preeclamptic placentas, an additional group of placentas from preterm deliveries without preeclampsia were also evaluated. The HIF protein expression was comparable in these preterm specimens and the normal term placentas. We conclude that protein expression of HIF-2α, but not of HIF-1α or -1β, is selectively increased in the preeclamptic placenta. The molecular mechanism(s) of this abnormality as well as the genes affected downstream are currently under investigation. To our knowledge, this is the first report of abnormal HIF-2α expression in human disease other than cancer.