OBJECTIVE: Our purpose was to develop fluorescence in situ hybridization to repetitive chromosome-specific sequences to detect chromosome aneuploidy faster than hybridization to unique targets or karyotyping. STUDY DESIGN: Aneuploidy involving chromosomes 13, 18, 21, X, and Y comprises 70% of chromosome abnormalities in 10- to 12-week fetuses, 95% of the phenotypically significant newborn chromosome abnormalities. Our improved 8-hour protocol used repetitive probes to label and count the number of these centromeric chromosome domains. RESULTS: This protocol correctly determined chromosome 13, 18, and 21 status in 50 of 50 unselected direct amniocyte samples and found abnormal patterns in 27 of 27 archived trisomy 21 cases. Altogether karyotyping confirmed 744 of 745 chromosome-specific repetitive sequence test results. CONCLUSION: This protocol rapidly tests abnormal fetuses and newborn infants in whom diagnosis is made at the initiation of labor or before urgent surgery when a cytogenetic result cannot be completed. (AM J OBSTET GYNECOL 1996;174:886-90.)