The results of a randomized clinical trial concerning the value of post-operative radiotherapy as an adjuvant to radical mastectomy, are presented in this manuscript. A total of 1090 patients have entered the study. In the first part a conventional roentgen unit was used, and in the second part a 60Co unit, with considerably increased dosage and altered treatment plan. No effect of post-operative radiation was demonstrated in Stage I regarding survival, disease-free survival, local recurrence, regional and distant metastases. In Stage II roentgen irradiation reduced the incidence of local recurrences and regional metastases, whereas the survival was not affected. Besides reduction of local and regional metastases 60Co radiation in patients with Stage II disease also reduced significantly the relapse rate as well as the cumulative survival rate up to 5 years. Thus, the results indicate that the radiation dose is of significance. Further analyses suggest, particularly in patients with medially located tumors who have 4 or more axillary lymph nodes involved, that the prognosis will be improved by post-operative radiotherapy in adequate doses.