It is proposed to determine if elective post operative irradiation given to patients with carcinoma of the rectum and colon with regional lymph node involvement (Stage Dukes' C) or adjacent organ involvement will result in a longer recurrence free interval and a higher survival than occurs with surgery alone. Evidence is accumulating from studies using pre-operative irradiation that doses of radiation of the order of 2000-4500 rads given in 2-5 weeks have a definite effect on the extent of disease while cures can be achieved with irradiation alone when higher doses are used. The value of post operative irradiation in patients with colorectal carcinoma with positive regional nodes are adjacent organ involvement has not been investigated. Moderate doses of radiation of the order of 4500-5000 rads delivered in 4 1/2-5 weeks may be effective in preventing clinical recurrence following surgical resection by controlling minimal disease, as has been shown to occur in other cancers. Therefore, it is planned to treat the patients accepted for this study with moderate dose wide field irradiation aimed at sterilizing post operative minimal disease, either residual or due to tumor spillage. We are confident that patients can tolerate the proposed treatment, since 16 patients with locally advanced colorectal cancer have successfully completed postoperative radiation therapy in our institution without significant morbidity or any mortality. In addition, we plan to treat the liver with a low dose of radiation since this organ is a frequent site of metastases in colorectal cancer. We hope to prevent the development of such metastases. Ten patients have had liver radiation thus far without any serious consequences.