Numerical aberrations of chromosomes can be detected by fluorescence in situ hybridization (FISH), using chromosome-specific probes. It is possible to observe this in solid tumors from which it is very difficult to obtain metaphase nuclei. The present study employed surgical specimens from 15 cases of colorectal carcinoma, all of which showed DNA diploidy. In the same samples, we analyzed chromosomal numerical aberration by FISH according to the method of Pinkel et al. Biotinylated DNA probes specific to chromosome #7, #11 and #17, were used. The hybridization spots were observed by fluorescent microscopy. As a result, the numerical aberrations of chromosomes detected by FISH were found in DNA diploid cases by FCM. They were trisomy and monosomy. These results indicate that FISH is useful to detect the chromosomal aberrations of DNA diploid cases.