To identify the effect of post-operative irradiation to the thyroid gland in patients with breast carcinoma. Seventy seven patients with partial or total mastectomized breast carcinoma who received routine irradiation therapy (Hockey stick method: supraclavicular, internal mammary lymph nodes, and chest wall irradiation with 5, 040 rads, divided into 30 treatments) were reviewed in terms of their ipsilateral thyroid gland response. All patients had the bilateral thyroid sizes measured annually by ultrasonography before and after radiation therapy. In the one-year follow-up group (n=77), 32 patients (41.5%) demonstrated decreased ipsilateral thyroid gland size after Hockey Stick irradiation therapy (p=0.428), in the two-year follow-up group (n=37), 26 patients (70.3%) demonstrated decreased gland size after Hockey Stick irradiation (p=0.001), and in the three-year follow-up group (n=21), 15 patients (71.4%) showed a decreased thyroid gland size (p=0.005). Most the patients with breast carcinoma (32/77 at the one-year follow-up, 26/37 at the two-year follow-up, and 15/21 at the three-year follow-up) after post-operative Hockey Stick irradiation therapy showed reduced ipsilateral thyroid gland size. Routine en face treatment of the supraclavicular lymph nodes, using the Hockey Stick method, should be reconsidered.