Large-aperture, dynamically focused ultrasonic imaging permits noninvasive, anatomic study of the eye at the millimeter level in the second and third trimesters of pregnancy. The authors report their observations of the hyaloid artery in 210 of 219 fetuses examined with this technique. This vessel is seen in fetuses of 20 weeks gestational age or less and regresses spontaneously at the start of the third trimester. The 210 subjects included 100 who were examined at gestational ages of 16-32 weeks or more and 85 healthy fetuses and 25 with pathologic findings at birth who were examined at 34 weeks gestational age to term. The presence of the hyaloid artery in the mid third trimester was uncommon in healthy subjects (less than 1%) and was not seen in any beyond 29.9 weeks gestational age. However, in nine of the 25 fetuses with abnormalities (five with trisomy syndromes), the vessel was seen at 30.8-36.8 weeks gestational age. The temporarily delayed regression of the hyaloid artery may occur with trisomy 21 syndrome and other forms of retarded brain development.