We report 11 cases with cancer of the thoracic esophagus developing after postoperative irradiation therapy for the breast cancer. Irradiation was done immediately after mastectomy in these cases and the irradiation field included the unilateral or bilateral parasternal region. They received a total dose ranging from 35 Gy to 60 Gy and the dose received to the thoracic esophagus was estimated from 10 Gy to 48 Gy. All cancer sites were involved in the irradiation filed. The latent intervals of 10 cases from radiation to the manifest of cancer ranged between 10 to 19 years. Among 4777 women undergone mastectomy for breast cancer between 1946 and 1980 in our hospital, 8 women (0.17%) developed cancer of the thoracic esophagus, whereas 5 (0.335%) out of 1534 women treated with mastectomy and radiotherapy with Linac between 1964 and 1980 developed cancer of the thoracic esophagus. Higher incidence of esophageal cancer in patients treated with surgery and radiation suggests that these cancers might be induced by radiation. Eight patients had esophagectomy and 4 patients of them received postoperative irradiation. They have survived from 9 months to 13 years. Two patients were controlled well by the irradiation alone. It is interesting that radiation therapy is sensitive to the possible radiation induced cancer of the thoracic esophagus. Follow up study should be directed to the possible development of second malignancy in patients who survive for a long time after radiation therapy.