• Large nuchal cystic hygromas were observed in five second-trimester aborted fetuses at autopsy. Two female fetuses with generalized edema were karyotyped as 45,X. One of these was the twin of a 46,XX normal female sibling. The association of generalized edema with large nuchal cystic hygromas was seen only in these two fetuses and represents strong phenotypic evidence of Turner's syndrome. However, the absence of hydrops was not a reliable indicator of normal karyotype. One fetus without generalized edema was karyotyped as 47,XY, +21, inv(9). The remaining cases had normal karyotypes. Placental histology was not useful in discriminating monosomy X from other conditions, but placental tissue culture was important in obtaining a cytogenetic diagnosis. Karyotyping is recommended in all cases of fetal cystic hygroma.