A 72-year-old man admitted to our hospital due to severe pain and drop hand in the left arm. MRI and CT of the head and neck at an outside hospital showed no abnormality. Neurological findings revealed distal weakness in the left upper extremity, vague pain between the radial aspect of the left hand and the middle of the digits 1–3, as well as brisk reflexes in the left extremities. Small masses were palpable in the posterior neck, the lower jaw, and the left neck. Clinically, left radial neuropathy and cervical radiculopathy were suspected. We performed ultrasound of the nerve roots, brachial plexus, and the radial nerve. There was a mass compressing the left brachial plexus from the caudolateral direction. Additionally, nerve swelling in the left arm was identified. Skin biopsy over the mass suggested metastatic adenocarcinoma. Chest CT scan showed a mass in the upper right lobe suggestive of a lung cancer. We concluded that the pain was due to radial neuropathy and upper and middle trunk disturbance of the left brachial plexopathy, of which neuromuscular ultrasound was useful in diagnosis.