Chromosome analysis were carried out on peripheral blood leukocytes of breast cancer patient during the irradiation therapy after unilateral simple mastectomy. The observations were made at intervals varying from one to 5 weeks during the therapy and one month after the completion of tile treatment. During the first and second weeks of treatment normal metaphase was noted and during the 4th and 5th weeks, there were no mitotic figures from the cell population. The chromosomal aberrations found after 3 weeks of treatment were, 11% of simple chromatid breaks, 7% of chromatid interchanges (translocations) and 8% of fragments. One month after the completion of the course of treatment showed a return of mitosis and that total chromatid breaks had decreased to 5%.