Conservative treatment has become a valid alternative to radical surgery in most cases of cancer of the anal canal and in selected cases of cancer of the low rectum. In this strategy interstitial curietherapy has an appreciable role to play. The results of a series of 369 patients followed more than 3 years indicate that implantation of Iridium-192 is effective not as sole treatment but as a booster dose 2 months after a course of external beam or intracavitary irradiation. The dose delivered did not exceed 20 to 30 Gy and the implantations were always performed in one plane using either a plastic template or a steel fork. Three groups of cases must be considered: (a) among 221 patients with epidermoid carcinoma of the anal canal, the rate of death related to treatment failures was 20% and among the patients cured more than 90% retained normal sphincter function. (b) In 90 patients with T1-T2 invasive adenocarcinoma of the rectum, Iridium-192 was carried out after four applications of contact X ray therapy. The rate of control was 84%. (c) In 62 elderly, poor risk patients with T2-T3 tumor of the low rectum initially suitable for an abdomino-perineal resection, a tentative extension of the field of conservation was made using a split-course protocol combining a short course of external beam irradiation at a dose of 30-35 Gy in 10 fractions over 12 days and an Iridium-192 implant. The rate of death due to treatment failures was 14.5% and among the patients controlled 97% had a normal anal function. These results show that implantations of Iridium-192 may contribute to the control of anal and rectal cancers and may spare many patients a permanent colostomy, but the treatment requires great care in patient selection, treatment protocol, technical details, and follow-up. This treatment policy must be conceived as a team work of radiation oncologists and surgeons.