Posterior cervical hygroma, once considered diagnostic of fetal Ullrich-Turner syndrome, is now recognized as being a nonspecific malformation found in a number of unrelated conditions. We have studied 61 fetuses of 10-23 developmental weeks from therapeutic and spontaneous abortions who had posterior cervical hygroma. We found that the group was indeed markedly heterogeneous, for both sexes were represented, and a variety of genetic and nongenetic disorders were present, such as trisomy 18, 21, and probably also 13; lethal multiple pterygium syndrome; Noonan syndrome; isolated congenital heart defect, isolated pterygium colli; and monosomy X (45,X). Unlike any of the other conditions, 45,X fetuses had a constant association of 3 defects--posterior cervical cystic hygroma, generalized subcutaneous oedema, and preductal aortic coarctation. Therefore, we suggest that this triad is diagnostic of lethal Ullrich-Turner syndrome in the fetal period and that it may be used reliably to diagnose the syndrome in fetuses when chromosome analysis is impossible. Further, since preductal coarctation of the aorta is found in only a small proportion of liveborn girls with Ullrich-Turner syndrome, but occurred in all aborted fetuses in our study, we suggest that this abnormality is related to the lethality of the 45,X chromosome constitution during fetal development.