Between 1968 and 1975, the authors studied at the Institut Curie the value of fine needle aspiration cytology in the follow-up of breast cancers, which have been previously treated with definitive irradiation (182 cases) or lumpectomy and post-operative irradiation (20 cases). The cytologic results have been compared to the pathologic aspects of the surgical specimens. The cytologic procedure was made when the tumor decrease was incomplete or too slow, or when, after a free interval, was noted in the irradiated breast a tissue reaction clinically and radiologically indefinite. The post-therapeutic investigation interested 202 cases of breast carcinoma. Compared to the histologic result, the cytologic results proved to be: -- identical. . . . . . . in 170 cases (84%) -- false benign . . . . . in 4 cases (2%) -- false malignant . . . . in 4 cases (2%) -- insufficient cell material in 24 cases (12%) The high rate of correct cytologic diagnosis and the very low rate of wrong results prove the value and interest of the fine needle aspiration cytology for residual tumor masses or for recurrences in irradiated breast cancers.