A success rate of about 90% has been achieved after primary operations for hyperparathyroidism, compared with 60% to 80% in most series of secondary operations. The present reoperative series involved 29 patients who underwent venous catheterization with blood sampling for the determination of intact parathyroid hormone before undergoing repeat parathyroid surgery. Blood samples were taken from the internal jugular veins, innominate veins, and superior caval vein. No attempt was made to perform superselective catheterization of the small neck and mediastinal veins. The reoperations were done by four surgeons who did 1, 2, 13, and 13 of the reoperations, respectively. In all patients, distinct step-ups in parathyroid hormone concentrations were found. On average, the gradient between the highest and lowest value was about 5. Close to the location of the step-ups, diseased parathyroid tissue was found in 27 of the patients. In two cases no parathyroid tissue was found, and these patients remained hyperparathyroid postoperatively. They had been treated by the surgeon who did only two of the operations. When the step-up was observed in the left innominate vein, we could not differentiate mediastinal from low cervical adenomas. No patient developed hypoparathyroidism. To avoid this complication, autotransplantation of diseased parathyroid tissue into the abdominal subcutaneous fat was done in nine patients. No case of recurrent laryngeal nerve paralysis occurred. The introduction of reliable assays for the analysis of parathyroid hormone can make selective catheterization unnecessary when localizing remaining parathyroid glands in patients with persistent hyperparathyroidism.