OUTLINE: Patients with stage III colorectal carcinoma (post surgical resection) receive irinotecan IV over 1 hour on days 1-5 and days 8-12. Treatment repeats every 3 weeks for 2 courses. Beginning on week 7, patients receive pelvic irradiation, leucovorin calcium IV over 4 hours, and fluorouracil IV over 5-15 minutes beginning 1 hour into leucovorin calcium infusion on days 1-5. Treatment repeats every 3 weeks for a total of 2 courses. Patients with no evidence of disease may repeat the above 12-week block of chemotherapy without pelvic irradiation up to 4 times over 1 year.