The causes of ipsilateral arm pain were analyzed in a consecutive series of 38 patients with breast cancer. A lesion of the brachial plexus was diagnosed in 17 patients, of whom 8 had tumor involvement, 5 had radiation fibrosis, 1 had lymphedema entrapment, and 3 had a probable transient neuritis of the plexus. In four patients, a cervical radiculopathy was found; two of these patients had a Horner's syndrome. A carpal tunnel syndrome was seen in four patients and could possibly be attributed to lymphedema in two patients. In five patients, a pericapsulitis of the shoulder joint was seen. Seven of eight patients with a postsurgical pain had a neuropathic pain related to damage of the intercostobrachial nerve induced by a postaxillary dissection. These diagnoses probably indicate the most common causes of ipsilateral arm pain in breast cancer. A postaxillary dissection pain seems the most frequent type of postsurgical pain in breast cancer.