Ten sets of diamniotic twins with discordant fetal growth (ultrasonographic estimated fetal weight difference > 25%) and abnormal amniotic fluid volume were assessed for fetal serum erythropoietin concentration (Epo). All 20 fetuses had a percutaneous umbilical blood sampling performed between 21 and 33 weeks gestation. Three sets of twins were dichorionic while 7 sets were monochorionic. In these 7 monochorionic sets a vascular communication between twins was demonstrated prenatally and postnatally. From 24 weeks gestation the smaller twin of the dichorionic group had a higher serum Epo than its larger cotwin; the larger cotwin's serum Epo was within normal limits for singleton controls. All fetuses with documented vascular communications had higher serum Epo concentration than singleton controls. These findings suggest that determination of fetal serum Epo in the large cotwin may be a clue to the diagnosis of vascular communications between twins with discordant growth.