Neuralgic amyotrophy is an inflammatory condition of the nerves of the brachial plexus, the cause of which is unknown. Clinically, it presents as acute, severe pain in the shoulder girdle, followed by paresthesia and flaccid paralysis of selected muscles in the upper limb. In its initial stages, the disease may easily be misdiagnosed as a musculoskeletal disorder. We describe a 38-year-old man who experienced acute, severe pain in both shoulders, and hypoesthesia and paresthesias in the right arm without preceeding trauma. The condition was first diagnosed as capsulitis of the shoulder joint, and the patient was treated with naproxen with no effect. Eight days after onset of the disease, flaccid paralysis occurred abruptly in the right biceps muscle and in the left spinati and serratus anterior muscles. Sensibility and muscle strength improved gradually without treatment during the following months.