Reversible branchial plexopathy has occurred in very low incidence in patients with breast carcinoma treated definitively with radiation therapy. Of 565 patients treated between Jan. 1968-Dec. 1979 with moderate doses of supervoltage radiation therapy (average axillary dose of 5000 rad in 5 wk), 8 patients (1.4%) developed the characteristic symptoms at a median time of 4.5 mo. after radiation therapy. This syndrome consists of paresthesias in all patients, with weakness and pain less commonly seen. The symptom complex differs from other previously described brachial plexus syndromes, including paralytic branchial neuritis, radiation-induced injury and carcinoma. A possible relationship to adjuvant chemotherapy exists, though the etiology is not well understood. The cases described demonstrate temporal clustering. Resolution is always seen.