From a total of 65 colorectal adenocarcinomas studied by cytogenetic methods, 33 were selected for the present study; in addition to other karyotypic anomalies, these 33 showed a loss of the short arm of chromosome 17. This loss was either the result of a deletion or rearrangement, or caused by the loss of a whole chromosome 17. The 17p- tumors were characterized by a high grade of karyotypic abnormality including a high incidence of cases with double minutes. A gain of chromosomes 2, 7, 19, and 20, and the loss of chromosome 18 and the Y-chromosome were the most frequent numerical anomalies associated with 17p-, as were structural changes of chromosomes 1 and 5. The most impressive difference in the pattern of proto-oncogene over-expression between the 17p- tumors and those without this anomaly was the significantly increased frequency of cases with c-erbB over-expression. Some significant, but also loose, associations were found between cytogenetic/oncogenetic and histopathologic or clinical features of these tumors. The patterns of genetic changes in cells of colorectal carcinomas may thus reflect the potential of the future development, rather than the present clinical features, of the respective tumor. Therefore, the character of the change seems to be more prognostic than diagnostic.