Neurologic complications from radiotherapy can be immediate or can occur many years after treatment. A known complication of radiotherapy to the supraclavicular and axillary lymph nodes is brachial plexus neuropathy. Although not a common injury, phrenic nerve dysfunction has been reported in association with radiation-induced brachial neuropathy. We describe a patient who developed asymmetric diaphragmatic weakness secondary to phrenic nerve paralysis 37 years after receiving mantle radiation for Hodgkin lymphoma. The patient did not have an associated brachial plexus neuropathy or a secondary malignancy involving the phrenic nerves. A radiation-induced injury was the most likely cause.