Between 1992 and 2002, 542 patients underwent a surgical treatment for hyperparathyroidism in our department. Twenty-three selective venous sampling procedures (SVS) were performed because of the failure of the other methods of diagnosis. These patients have recurrent or persistent hyperparathyroidism. Noninvasive methods of topographical diagnosis have failed or they have given contradictory results. In our experience, the surgeon needs a precise localization of the pathological glands in these difficult cases. In our series of SVS, specificity was 85.7% and sensitivity was 94.7%. Our results show that a high postoperative gradient of parathyroid hormone in the internal thoracic veins indicates an ectopic pathological gland in the thymus. A high gradient in a vertebral vein indicates a pathological superior parathyroid gland, usually in a retro-esophageal position.