The objective of this study was to test the hypothesis that in discordant twins, the smaller infant has higher absolute nucleated red blood cell (RBC) count than the larger sibling. We compared absolute nucleated RBC counts, hematocrits, absolute leukocyte counts, absolute granulocyte counts, absolute lymphocyte counts, RBC counts and platelet counts obtained in the first 12 hours of life in 30 discordant twin sets. The smaller infant had a higher absolute nucleated red blood cell count and lower platelet counts than its larger sibling. Platelet counts correlated inversely with absolute nucleated RBC counts ( R(2) = 24.5%; p < 0.001) and absolute nucleated RBCs correlated directly with percent intertwin weight differences ( R(2) = 17.8%; p = 0.02). In discordant twins, the smaller infant has higher absolute nucleated RBC count and lower platelet counts than the larger sibling. We speculate that the hematologic alterations in the smaller fetus are linked to relative fetal hypoxia.