Objective:To examine the changes in hematologic and coagulation indices in the surviving twin when the co‐twin dies because of the twin‐twin transfusion syndrome.Methods:Fetal blood was obtained by funipuncture in seven surviving twins upon the death of their co‐twins. Five of them were monochorionic. In one case at 32 weeks' gestation, two repeated funipunctures were done in both twins before and in the surviving twin after the death of the co‐twin. Fetal blood was examined for blood coagulation factors as well as complete blood counts.Results:Although coagulation factors were not abnormal, three of the five monochorionic surviving twins had cerebral abnormalities postnatally. The fetal blood profile revealed anemia in the surviving twin, especially in the cases in which funipunctures were performed within 24 hours after the co‐twin's death. This demonstrates that acute anemia in the surviving twin was induced by hemorrhage from the larger to the smaller twin at the time of death.Conclusion:Following the death of one twin, morbidity in the surviving twin can be produced by hypotensive ischemia of the brain due to hemorrhage through placental vascular anastomoses.(Obstet Gynecol 1994;83:975‐8)