FISH analysis of uncultured interphase amniotic fluid cells from a male fetus revealed two signals using an alpha-satellite X-chromosome DNA probe. One of the signals was much smaller than the other. It was subsequently shown that the normal sized signal was located on the X chromosome and the smaller signal was located at the centromere of chromosome 19. This hybridization pattern was confirmed in the newborn infant and in his phenotypically normal father. The use of alpha-satellite DNA probes on interphase cells could result in false-positive errors due to rare variants such as the X-chromosome alpha-satellite found on chromosome 19 in our patient.