DNA cytofluorometry and fluorescence in situ hybridization (FISH) with centromeric repetitive probes of chromosomes 1, 7, 11, 17, X and Y were used to detect numerical chromosomal aberrations in 13 squamous cell carcinomas of the tongue. In 4 of the 6 DNA-diploid tumors examined, significant numerical aberrations in at least 1 of the chromosomes were detected. The main line of 1 tumor showed trisomy 1 and the other 3 tumors had significant sublines that showed trisomy or monosomy of the 1 or 2 chromosomes examined. No common specific aberrations were detected in these tumors. Cytofluorometrically we found polyploidy in all 4 of the tumors with numerical chromosomal aberrations. These results suggest that subpopulations which are aneuploid at the chromosome level arise preferentially from DNA-diploid tumors with polyploidy before the ploidy of the main line shifts to overt DNA-aneuploidy. In all of the 7 DNA-aneuploid tumors examined, we detected numerical chromosomal aberrations. Most of the aberrations were thought to occur after tetraploidization because a gain in chromosomal copy number including tetrasomy was common. In 3 DNA-aneuploid tumors, however, the main line showed disomy 11, whereas the other chromosomes examined had 3 or more copies. In 1 of the 3 tumors there was a significant subline of monosomy 11. In these 3 tumors disomy 11 may have been preceded by a DNA-diploid stage with monosomy 11 before tetraploidization.