Numerical aberration of chromosome 17 of 14 cases of colorectal carcinoma with multiple primary cancer (: multiple cancer) was compared with that of 35 cases of colorectal carcinoma without any other cancer (: single cancer). Fluorescence in situ hybridization with p17H8 was performed on touch smear from fresh materials. The proportion of aneusomy 17 (NCAI: numerical chromosome aberration index) in multiple cancers was significantly higher than that of single cancers (37.7 +/- 10.5% VS 46.1 +/- 8.0%; p < 0.01). Although NCAI of single cancers conformed to cancer progression (26.1 +/- 4.7% in Dukes A, 33.1 +/- 7.1% in Dukes B, 39.9 +/- 6.9% in Dukes C, and 45.7 +/- 12.0% in Dukes D), that of multiple cancers was high in all stages (44.7 +/- 7.3%, 44.4 +/- 6.8%, 50.4 +/- 11.2%, and 49.6 +/- 5.6%, respectively). Furthermore, the multiple numerical aberration of chromosome 17 in multiple cancers was more often than that of single cancers (64.3% VS 22.9%; p < 0.01).