We measured DNA content by flow cytometry and determined the number of chromosomes 7, 17 and 18 by fluorescence in situ hybridization using DNA probes in 38 sporadic colorectal adenocarcinomas including 14 early cancers. Based on DNA ploidy and the copy number of chromosome 18, colorectal cancers were divided into two groups. In early cancers (mucosal and submucosal adenocarcinomas), monosomy 18 with DNA hypotriploidy was detected exclusively in 6 sessile tumors without adenoma portions. Seven of the remaining 8 tumors in which adenoma portions coexisted manifested disomy 18 and DNA diploidy or near diploidy. These tumors were considered to occur along the adenoma-carcinoma sequence. In contrast, monosomy 18 was accompanied by DNA diploidy or near diploidy in advanced cancers penetrating the muscularis propria. In advanced cancers, DNA hypertriploidy was linked with disomy 18 and chromosomes 7 and 17 were trisomic or tetrasomic. Observations indicate at least two different cytogenetic pathways in colorectal carcinogenesis. According to these findings, two thirds of advanced cancers were estimated to originate from adenoma, and the others were not.