A 201Tl-chloride sacn was performed on a patient with primary hyperparathyroidism. 201Tl-chloride accumulated in the region of the right lower thyroidal pole and a diagnosis of parathyroid adenoma was confirmed intraoperatively. 201Tl-chloride imaging of a hyperfunctioning parathyroid gland seems to be useful for the detection and preoperative localization of a parathyroid adenoma.