Persistence of elevated alphafetoprotein (AFP) levels and the presence of an acetylcholinesterase (AChE) band in amniotic fluid have been reported to occur up to 11 weeks following intrauterine fetal demise (IUFD) of one twin (Bass et al., 1986). We now report a case where such prolongation of these findings was observed in a case of unrecognized monochorionic, monoamniotic twinning, in which case cord entanglement resulted in IUFD at an estimated 10-12 weeks and 25-26 weeks. The fetus suffering early demise (Fetus B) had multiple congenital anomalies, including a neural tube defect. The presence of this defect and/or fetal demise and bleeding into the amniotic sac is entertained as continuing sources of documented elevated AChE and AFP 9-11 weeks after the initial fetal death. We re-emphasize the possibility of unrecognized twinning as a cause of abnormal maternal serum and amniotic fluid study results in the face of one apparently normal fetus.