Cytogenetic analysis of short-term cultures from 52 colorectal carcinomas revealed a normal karyotype in 13 and clonal chromosome aberrations in 39 tumors. In the abnormal group, 13 tumors had simple numerical changes only, whereas 26 had at least one structural rearrangement with or without concomitant numerical changes. The most common numerical abnormalities were, in order of decreasing frequency, +7, −18, −Y, +8, +13 and −14. The most common structural rearrangements affected, again in order of decreasing frequency, chromosomes 8, 1, 6, 7, 17, 3, 11, 13, 14, 16, 2 and 10. The chromosome bands most frequently involved in the structural changes were 8q 10, 17p11, 11q13, 8p11, 6q21, 7p15, 7q36, 12q13, 13q10, and 16q13. The most frequent genomic imbalances brought about by the structural rearrangements were losses from chromosome arms 8p, 1 p, 6q, 17p, 7p, and 16q, as well as gains of 7q, 8q, 13q, and 11 q. A statistically significant (p < 0.05) correlation between the karyotypic pattern and tumor grade was found, with the poorly differentiated carcinomas generally having more massive chromosomal abnormalities.