Many studies in clinical oncology rely on hospital-derived patients. Hospitals vary in the proportions of patients from the local catchment area vs. those from more distant places, of whom a larger proportion are presumably referrals. To study the differences between these two types of patients, we analyzed 1,245 colorectal cancer patients seen at a large urban medical center over a seven-year period. Three hundred ninety-eight patients were from the local community (32 percent), 489 were from the extended community (39.3 percent), and 358 from more distant communities (28.8 percent). The patients from the local community tended to be older and from minority ethnic groups. In addition, the local community patients were more likely to have advanced disease at the time of presentation. The grade of the tumor and its site distribution within the large bowel were similar for the three groups. After adjusting for age, sex, race, and stage of disease, the survival was somewhat better for the distant community patients as compared with the local and extended communities (P less than 0.02). Overall, in our patient population, the distant patients tended to have more favorable socioeconomic factors and less advanced disease, and these differences may account, in large part, for a better prognosis for these patients. Particularly in large cooperative trials, studies may need to take into account the respective proportions of local community and geographically distant patients in analyzing and generalizing treatment outcomes.