Mediastinal thymic cysts are usually asymptomatic and found incidentally on a routine chest roentgenogram. Rarely, they may cause symptoms of vascular obstruction. A 55-year-old woman presented with intermittent swelling in her left neck. The swelling was positional and was worse while supine and disappeared while upright. Evaluation revealed a thymic cyst causing extrinsic compression of the left brachiocephalic vein. The cyst was resected with complete resolution of the left neck swelling.