Biopsy specimens were obtained under direct vision at the time of cesarean section from 42 patients (33 preeclamptic and nine normotensive patients) and from three hysterectomies (all in normotensive patients). Mean gestational age was 32.8 +/- 0.9 week (range, 26 to 40 weeks) in women with preeclampsia and 36.1 +/- 1.1 weeks in normotensive women (range, 32 to 41 weeks). Tissues were obtained from the central placental bed and from nonplacental sites. Specimens were examined histologically and by electron microscopy. Ultrastructural changes in small vessels, primarily venules, were compared with preeclamptic blood pressure. Extensive ultrastructural endothelial injury was found consistently in both site and nonsite areas in all of the specimens from women with preeclampsia but not in normotensive women (p less than 0.0001). There was no apparent correlation between the type or degree of endothelial damage and maternal hypertension. The same types and relative severity of specific vascular injury were present in both placental and nonplacental sites. Endothelial and derivative vascular injury occurs more or less uniformly in the uterus in preeclampsia, especially along the boundary zone between maternal and fetal tissues.