During a 6-year period (1985-1990), blood karyotyping was performed in 44 fetuses with septated, bilateral, dorsal, cervical cystic hygromata. This condition constitutes a different entity from nuchal oedema. There were 33 (75%) chromosomal abnormalities, including Turner's syndrome (n = 31), trisomy 18 (n = 1) and trisomy 21 (n = 1). Congenital heart defects (CHD), mainly coarctation of the aorta, were present in 15 of the fetuses with Turner's and in 1 of the chromosomally normal fetuses. The incidence of CHD was higher in the fetuses with ultrasonographic evidence of moderate/severe hydrops (41%; 13 of 32 cases) than in those with mild or no hydrops (25%; 3 of 12 cases). Although both the biparietal diameter (BPD) and femur length (FL) were reduced in all fetuses, the FL to BPD ratio was below the 5th percentile in 29 of the 33 (88%) chromosomally abnormal fetuses, but in only 4 of the 11 (36%) chromosomally normal ones. In the chromosomally normal group, 3 of the fetuses had multiple pterygium syndromes, and in such cases the risk of recurrence may be high. In contrast, in the group of mutant chromosomal disorders with monosomy or trisomy, the risk of recurrence is in the order of 1%.