With the purpose to study the changes of long-chain fatty acid composition in the serum of the breast cancer patients after radiation therapy, total fatty acids in the serum of the patients with breast cancer, both before and after the radiation teatment, were separated and identified by the the gas-liquid chromatography. The results of the study may briefly be summarized as follows. 1. The total fatty acid composition in the serum at the time when the tumor has grown smaller approaches the level of normal persons in contrast to the level before the radiation therapy. 2. It is considered that the radiation therapy is a success when the polyunsaturated fatty acids, especially linoleic and arachidonic acids are increased and saturated fatty acids are decreased.