A sessile adenoma of the left flexure of the colon was studied after surgical colectomy. Specimens were obtained for complete histologic evaluation. The tumor consisted of glandular tubes with decreased mucin production and a papillary structure on the luminal aspect. The muscularis mucosa was not involved; there was no carcinomatous focus. Cytogenetic study was carried out on 56 cells; none was normal, 77% were hyperdiploid (52-87 chromosomes), 16% were hypodiploid (18-39 chromosomes), and 7% were paradiploid. The supernumerary chromosomes were chromosomes #3, #6, #13, #19, and #20; chromosome #18 was missing in 80% of the cells. A marker for chromosome #1 resulting from a q21.1-q21.2 break with inversion of the centromere-bearing segment (pter-q21) was observed in 58% of the cells. Twenty-five percent of the cells had double minute chromosomes. Despite the histologically benign nature of the tumor, all the cells showed significant cytogenetic aberrations, some of which are considered to be markers of neoplastic transformation (polyploidy, double minutes, chromosome #1 marker).