It has been established that 60% of infants with Down's syndrome have hypoplasia of the middle phalanx of the fifth digit. To determine whether this would be a useful prenatal sonographic sign for Down's syndrome, we measured the middle phalanx of the fifth and fourth digits in 1,032 fetuses between 15 and 20 weeks gestational age at the time of amniocentesis, prior to any knowledge of the karyotypes. A ratio of the middle phalanx of the fifth digit over the middle phalanx of the fourth digit was calculated, and the median ratio for the 1,024 normal fetuses was 0.85. There were eight fetuses who had trisomy 21 by karyotype and their median ratio was 0.59 (P = .04). Of the eight fetuses with Down's syndrome, seven had ratios below the normal population median. If an arbitrary cut-off point is used at a ratio of 0.70, 6/8 (75%) of those with Down's syndrome would be identified, as well as 18% of normal fetuses (positive predictive value = 3.2% in this group). Although we do not suggest that this ratio be used alone as a screening test for Down's syndrome, these findings confirm the presence of a small middle phalanx in fetuses with trisomy 21 as early as 15 to 16 weeks and may be a useful adjunct to the several already reported sonographic signs in the fetus at risk for Down's syndrome.