Background and aim: There are no reports of quantitative evaluation of metabolic changes in the mammary gland after radiation therapy. The purpose of this study was to investigate metabolic changes in the ipsilateral mammary gland after radiation therapy and to evaluate the late radiation effects. Patients and methods: Metabolic changes of the ipsilateral and contralateral mammary glands were retrospectively evaluated by F-18 fluorodeoxyglucose (FDG) PET/CT after whole breast irradiation following lumpectomy in five breast cancer patients. The maximum standardized uptake values (SUVmax) of the ipsilateral mammary gland (S1) and the contralateral mammary gland (S0) were measured. The relative SUVmax (5 S1/S0) was measured immediately before, one year after, three years after, and five years after radiation therapy. Results: The SUVmax of the ipsilateral mammary gland decreased gradually, but there was no decrease in the SUVmax of the contralateral mammary gland. Relative SUVmax values before, 1, 3, and 5 years after radiation therapy were 1.33 +- 0.20, 1.13 +- 0.35, 0.99 +- 0.26 and 0.62 +- 0.10, respectively. Relative SUVmax value at 5 years was significantly lower than before radiation therapy (P < 0.01). Conclusions: 18F-FDG PET/CT may be useful in assessing mammary gland function and predict lactation disorders in advance.  Kulcsszavak: breastfeeding, metabolism, radiation injury, adverse events, functional imaging