Schwanomas originating from the brachial plexus, although rare, may be mistaken for another type of tumour. A 40 year-old woman, who had been treated years earlier for a breast adenocarcinoma, showed in the 5-year follow-up magnetic resonance examination a localized lesion in the right brachial plexus. The presumptive radiological diagnosis was a metastasis from the primary adenocarcinoma. Following surgical access via the right axilla, a well-circumscribed mass in the brachial plexus was detected. Under intraoperative electrophysiological guidance, the lesion was observed to depend on the ulnar nerve and its complete resection was possible without compromising nerve function. Histological findings indicated a schwannoma thus ruling out the presence of metastasis. The postoperative development was uneventful and six years after surgery, the patient is to date tumour-free. To the best of our knowledge, this is the first report of a brachial plexus schwannoma arising from the ulnar branch. Intraoperative electrophysiological monitoring is essential for a good surgical outcome.