The chromosomes of 15 primary intestinal tumors were analyzed with a banding technique. Of the 15 tumors, 12 had some chromosomal abnormalities (8 with numerical changes and 4 with both numerical and structural abnormalities) and in the remaining three no karyotypic abnormalities were found. No common marker chromosomes were seen among the various tumors and no two tumors with chromosomal changes and identical karyotypes, though some chromosomes were involved more often than others. Excessive chromosomes in the primary tumors were usually due to extra chromosomes in the following groups (numbers of tumors involved are shown in parenthesis): No. 8 (7), No. 13 (4), No. 15 (4), No. 17 (6) and No. 21 (6). On the other hand, chromosomes losses, though much less frequent, involved chromosomes No. 5, No. 6, No. 7, No. 10 and No. 16. Most of the tumor cells with chromosomal changes were hyperdiploid and usually contained less than 60 chromosomes. Only one tumor contained hypodiploid cells. The cytogenetic data presented on primary intestinal tumors indicate that they consist primarily of numerical changes, relative infrequency (when compared to metastases) and small number (1-4) of markers.