Of 1,556 patients treated for various malignant disease in the Radiological Research Institute, office ofAtomic Energy during a period of three years from November 1963 to October 1966 inclusive, 110 had carcinoma ofthe breast (7.1%) 88(80%) of the 110 patients with carcinoma of the breast had completed a course of treatment. On22 patients(20%), the radiotherapy was discontinued for various reasons. This study was based on an analysis ofthe former group who had completed a course of Cobalt-60 teletherapy during this period of time. It is difficultto assess the result of the therapy in terms of survival rate because of the patients' poor cooperation for thefollow-up study. It, however,has been our experience that patients with recurrence of the disease nerver failed toreport. This made it possible to arrive at a fairly satisfactory estimation of the recurrence rate. As 66(75%) ofpatients in the study had either radical or simple mastectomy prior to radiotherapy, while the remaining 22(25%)had no operation, we divided them into two groups of post-operative radiotherapy group and radiotherapy alonegroup. Whether treated operatively or not, prior to radiotherapy, all the patients in both groups had the sameradiation consisting of a total tumor dose of 5,000 Rad in 5 weeks to the breast proper and lymph node bearingareas. The discrepancy of the recurrence rate between the two groups cannot be made for certain for these patientsreferred from other hospitals, where the clinical classification of the disease vary from one hospital to another(over 85% of the patients were referred from the other hospitals). There have been total of 10 recurrences in thestudy of 88 patients with a recurrence rate of 11.3% and 6(9.1%) of 66 patients with post-operative radiotherapydeveloped recurrence as against 4 (18%) in 22 patients with radiotherapy alone. Recurrence in the latter group wastwice as frequent as in the former. Although Haagensen and Stout's rigid criteria of operability was not fulfilledin some case of post-operative radiotherapy group prior to operation, post-operative radiotherapy showed lessrecurrence rate as compared with that of radiotherapy alone group. Terefore, we feel that post-operativeradiotherapy is significant for the treatment of an inoperable carcinoma of the breast as reported in theliteratures.