The objective of the study was to explore the natural history and clinical significance of lateral neck cysts during the early second trimester of pregnancy. A survey was conducted of pregnant women at 12-15 weeks' gestation who presented at the ultrasonographic unit between January 1991 and December 1994. During the 4-year period, of the 1500 fetuses scanned, 42 fetuses with lateral neck cysts were detected by high-resolution transvaginal ultrasonography. Twenty-six of the 42 fetuses were seen to have isolated lateral neck cysts, and 16 demonstrated a combination of nuchal thickness of > or = 4 mm and lateral neck cysts. Natural history, fetal karyotype and pregnancy outcome were compared between these two groups. None of the fetuses with isolated neck cysts had an abnormal karyotype, and all cysts resolved spontaneously at 16-20 weeks. No congenital abnormalities were found among the 26 cases. Four chromosomal abnormalities (three trisomy 21 and one monosomy XO) were detected among the 16 fetuses with combined lesions, resulting in termination of pregnancy in all. Among the remaining 12 fetuses, spontaneous resolution of the neck findings occurred between 16 and 20 weeks' gestation. The finding of isolated lateral neck cysts in the early second trimester is not associated with increased risk of aneuploidy. However, the combination with nuchal thickness of > or = 4 mm should prompt genetic counselling and consideration of karyotyping, since this delay in maturation of the cervical jugular lymphatic communication may be associated with chromosomal aberrations.