Background and purpose To contribute to the question whether the risk of radiation-related brachial plexopathy increases, remains constant or decreases with time after treatment. Patients and methods Between 12/80 and 9/93, 140 breast cancer patients received supraclavicular lymph node irradiation using a telecobalt unit. Total dose was 60 with 3Gy per fraction at a depth of 0.5 cm and 52 with 2.6Gy per fraction to the brachial plexus at a depth of 3 cm. Twenty-eight women received chemotherapy, 34 tamoxifen. Brachial plexopathy was graded using a modified LENT-SOMA score. Actuarial complication-free survival and overall survival were obtained from Kaplan-Meier analysis. The impact of chemotherapy or tamoxifen was tested using the χ2 test. The annual incidence of radiation-related brachial plexopathy was assessed by exponential regression as described by Jung et al. [Radiother Oncol 61 (2001) 233]. Results Actuarial overall survival was 67.1% after 5 years, 54.0% after 10 years, 49.9% after 15 years, and 44.0% after 20 years. In 19/140 patients, brachial plexopathy grade≥1 occurred after a median interval of 88 (30-217) months. The percentage of patients being free from plexopathy was 96.1% after 5 years, 75.5% after 10 years, 72.1% after 15 years, and 46.0% after 19 years, respectively. A significant impact of type of surgery, chemotherapy or tamoxifen was not observed. The annual incidence of brachial plexopathy was 2.9% for grade≥1 lesions and 0.8% for grade≥3 lesions. The rates did not change significantly with time. Conclusions The risk of brachial plexopathy after supraclavicular lymph node irradiation in breast cancer patients remains constant for a considerable portion of the patient's life.