Neurogenic thoracic outlet syndrome (TOS) is caused by compression of the lower brachial plexus usually by a cervical rib or a fibrous band. We describe a 16-year-old girl with weakness and wasting of her right hand, which progressed over the ensuing years. She had been a competitive long distance freestyle and butterfly swimmer since age 8 years. A neurological exam at age 20, revealed severe atrophy and weakness of all intrinsic right hand muscles, more so of the thenar muscles, and hypesthesia along the ulnar aspect of the hand and forearm. EMG, which showed a severe chronic axon loss lower trunk brachial plexopathy with minimal fibrillations, was typical for classic neurogenic TOS. Chest and cervical spine X-rays and MRI of the cervical spine were normal. A supraclavicular exploration confirmed the absence of a cervical rib or band. The lower trunk was thickened under the scalenus anticus which was sectioned. Neurolysis was also done. She was advised to abandon swimming. A clinical and EMG follow-up 2.5 years later showed no significant changes. This is the first case of true neurogenic TOS caused by scalenus anticus compression occurring in a competitive swimmer.