Specific loss of heterozygosity of chromosome 18 has been observed frequently in advanced colorectal carcinoma and is closely associated with its development. We investigated the prevalence of numerical aberrations of chromosome 18 in 44 specimens of colorectal carcinomas, using fluorescence in situ hybridization. We also examined the relationship between aneusomy of chromosome 18 and the clinicopathological features of these tumors. Aneusomy of the specimens (monosomy and polysomy) was determined when the same aneusomic population was detected in more than 15% of the nuclei. The frequency of monosomy and polysomy of chromosome 18 in colorectal carcinomas was 43% (19/44) and 29% (12/44), respectively. The prevalence of monosomy and polysomy 18 was significantly higher in cancers with invasion exceeding category T2 compared with T1 (P < 0.01), and with tumor size exceeding 20 mm in diameter compared with tumors less than 20 mm (P < 0.05). However, the prevalence of aneusomy 18 was not associated with other clinico-pathological features. The mean survival period and the 5-year survival rate after operation in patients with aneusomy 18 was not different from findings for those with disomy 18. Our results indicate that aneusomy of chromosome 18 is associated with the development of colorectal carcinoma; however, it is not a useful indicator of postoperative prognosis.