A 58-year-old woman presented with a palpable mass at the base of the left neck which she had first noticed 12 years previously, shortly after abdominal surgery. The mass had progressively enlarged, resulting in dysphagia, dyspnea and occasional pain. Imaging showed a septated but otherwise simple cystic mass extending into the mediastinum and containing lymphocytic fluid on aspiration. A diagnosis of the rare entity of a thoracic duct cyst with supraclavicular extension was made. The patient opted for percutaneous sclerotherapy of the lesion, which was performed using glacial acetic acid. This resulted in complete resolution of the mass with one treatment. After exclusion of other causes of cystic left supraclavicular masses including cystic neoplasms and pseudoaneurysms of the carotid or subclavian arteries, direct puncture sclerotherapy can be safe and effective.