A fluorescence in situ hybridization (FISH) protocol to simultaneously enumerate chromosomes 13, 18, 21, X, and Y in interphase cell nuclei for application in preimplantation genetic diagnosis (PGD) of aneuploidy was tested. Strict scoring criteria were developed to minimize recording errors. The protocol used optimized probes for chromosome-specific DNA repeat and single-copy loci and showed a significantly higher efficiency (95%) than previously published protocols. The other purpose of this study was to differentiate between two signals originating from a single split target or from two targets in close proximity. These criteria were based on the FISH results obtained from the analysis of all or most of the cells from 50 chromosomally normal or mosaic human embryos, 20 aneuploid embryos, and five polyploid embryos donated for research. Subsequently, 183 human embryos underwent PGD of aneuploidy in one of their cells. In 64 embryos that were not transferred back to the uterus, the rest of cells were also analyzed, and the previous results were confirmed in 91% of these embryos, with 0% (0/21) of the embryos classified as normal embryos being abnormal and 14% (6/43) of the embryos classified as abnormal being normal. Compared to previous protocols, these criteria minimize the risk of transferring abnormal embryos after PGD analysis.