[en] This study investigates the changes in the Ki67 labeling index during radiotherapy (RT) of seven patients with primary breast carcinoma, one patient with metastatic bronchial squamous cell carcinoma and one patient with a para-umbilical deposit of a large bowel adenocarcinoma. The material was taken by fine needle aspiration either from the primary tumors or the metastases. In four patients with primary breast carcinoma, we observed a drop in the Ki67 labeling index after 24 h (2 Gy), but an increase after 18 days (26 Gy). In one patient the pattern was different. An increase after the initial fraction was followed by a decrease after 18 days of radiotherapy. In two patients with primary breast carcinoma, where the second sample was aspirated after 72 h (3 fractions of RT, 6 Gy), the values of Ki67 indexes before RT were higher than after RT. In two patients where the material was aspirated from metastatic deposits, we observed an increase of Ki67 labeling index after 24 h (2 Gy). Possible explanations, including repopulations following the suppression of the synthesis in the originally active clonogens after radiation injury, are discussed. (author)