Conventional X- and Y-chromatin and fluorescent in situ hybridization (FISH) analysis based on X- and Y-chromosome specific probes were conducted from buccal smear, on 15 normal males, 15 normal females, and 9 cases suspected of sex chromosome anomalies. The proportion of X- and Y-chromatin in normal females and males was 12% +/- 3% and 51.5% +/- 4.9%, respectively, by the conventional X- and Y-chromatin procedure. The CEP-X/Y analysis by FISH for the same specimens provided a proportion of 98.8% +/- 0.7% cells with XX signals in the normal females and 99.8% +/- 0.4% cells with XY signals in the normal males. The FISH method was superior to the conventional procedure in nine cases suspected of sex chromosome anomalies, including one case of mosaicism. The results of CEP-X/Y will sometimes be false; it will not detect structural anomalies of sex chromosomes, and it is not intended to detect low level mosaicism. However, the test is useful for rapid screening of sex chromosome aneuploidy at a fraction of the cost for chromosome analysis. The FISH test is also appropriate to detect tissue specific sex chromosome mosaicism, especially if it is relatively high. This FISH test is best used as an adjunct to chromosome analysis whenever possible.