This is an open-label, single center, phase I study designed to determine the MTD or oral topotecan as a radiosensitizing agent in the treatment of rectal cancer patients. Sequential cohorts of three patients will be given increasing doses of oral topotecan and fixed doses of concurrent radiation (45GY) over five weeks. The starting dose of oral topotecan is 0.25mg/m2 to be concomitantly administered with radiation (45Gy) x 5 days every week unless the radiation is interrupted for Holidays/Weekends or toxicity requiring treatment delays occurs.