- Background. Primary true isolated metastasis in a peripheral nerve trunk is considered a rare phenomenon. Several routes of tumor invasion of the peripheral nervous system from different types of cancer and in various anatomic locations have been described in the literature; however, the brachial plexus is an uncommon site of such blood-born metastases. Methods. Five patients with severe brachial plexus neuropathy without obvious external signs of tumor masses and no detectable regional lymph node involvement underwent exploration of the plexus. Results. Isolated metastatic deposits were identified inside the nerve trunks. Radiation therapy to the area of the lesion provided good symptomatic relief to all patients and local arrest of the disease to four of five. Conclusions. Despite its rich vascularity, the Peripheral nerve has relative resistance to metastatic spread because there are blood-nerve barriers. This barrier should be investigated further in a tumor model. Cancer 1995;76:1829–32.