The characteristics of colon cancer tumors diagnosed in patients seen at hospitals participating in the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) Program and at Comprehensive Cancer Centers (CCC's) belonging to the Centralized Cancer Patient Data System (CCPDS) are compared. There were identified among cases diagnosed between July 1, 1977 and December 31, 1978, the first 18 months of registration for the CCPDS centers. A higher proportion of CCPDS colon tumors were diagnosed in black patients, 15.4% versus 6.8% for SEER, reflecting the urban location of many CCC's. The CCPDS has proportionally fewer patients aged 75 years or older, and a median age of 67.5 years versus 70 years for the SEER cases. Although surgery alone was the major form of therapy for both CCPDS and SEER patients with colon tumors, higher percentages of CCPDS patients than SEER patients were treated by chemotherapy alone or by modalities other than surgery, chemotherapy, and radiotherapy, particularly those with later stages of the disease. Few disagreements existed between the 2 groups in distribution by segment of the colon, stage, and histologic type. Few differences were found that would render invalid future comparative analyses of patient survival between the two data systems once adequate follow-up information is available. Such an evaluation may be a valuable instrument in measuring whether improvements in cancer patient management being developed at CCC's are, in fact, "filtering down" to the general series of colon cancer patients.