The Immune competence of 16 breast cancer patients treated with radical mastectomy was evaluated by lymphocyte counts, delayed hypersensitivity skin tests, and in vitro response of blood lymphocytes to phytohemagglutinin. Twelve patients who received postoperative irradiation were evaluated before and immediately after therapy. Four nonirradiated patients were studied during similar time intervals. The apparent degree of immunosuppression produced in the locally irradiated patients was surprisingly marked. Postmastectomy patients not receiving radiotherapy did not show these immunological defects during similar observation periods. It is postulated that this disruption of the biologic balance between host and tumor could be a relevant factor in the prognosis of the cancer patient.