The serum calcium level measured in the research laboratory was 10.8 mg/100 ml or more in all of 73 patients with primary hyperparathyroidism, though many of the values were normal when measured in the hospital laboratories. Postoperatively, all patients successfully operated on showed an immediate increase in renal phosphorus reabsorption to 95% to 100%, and a decrease, which averaged 54%, in urinary hydroxyproline levels. Seven patients who had hyperplasia had no clinical or laboratory features distinguishing them from those with adenomas. Uremia, an ominous sign, was found in six patients, five of whom died.