Successful cytogenetic analysis was performed on tumor material from 26 patients with resectable colorectal cancer. 9 women and 17 men, aged 43 to 92 years, median 67 years. Clonal anomalies were found in twenty patients; five tumors showed mainly slight numerical changes such as trisomy 7 and loss of Y (2 cases). The remaining 15 tumors had highly complex karyotypes. The mainline was near diploid in six cases (5/6 tumors of the proximal colon), near triploid in four and near tetraploid in five tumors. Loss of chromosomes was most frequently observed with chromosomes 2, 5, 18, 20, and Y, the most frequently gained chromosomes were 7, 8, 13, 15, and X. Structural aberrations affected all chromosomes, except Y. The most frequently rearranged bands were 5q21, 7p15, 9p21, 13q11, 16p12, 17p13, 18q21, 21q11. Anomalies of chromosomes 5, 17, and 18 occurred concomitantly in 9/20 patients. All patients with deletions of 17p (n = 6) had near tetraploid karyotypes with high cell to cell variability and a median of nine structural aberrations (p < 0.007); four of them presented with parenchymal metastases at the time of surgery. Tumors of the proximal part of colon were with one exception diploid or near diploid, but no specific pattern of aberrations was detectable. However, it appears noteworthy that of the six patients with tumors of the ascending colon, three tumors had deletions at 16p12 and the affected patients had a short duration of survival. The tumor karyotypes of patients with parenchymal metastases revealed a trend to greater complexity of numerical and structural aberrations. Changes involving 8p22 or loss of chromosomes 8 were found in tumors of all parts of the colon and potentially associated with an unfavorable prognosis (4/7 decreased patients showed such changes).