A simple, instant, and reliable test called the Density Test, based on the difference in density between the normal and the diseased parathyroid tissue, was performed intraoperatively in 73 patients with primary hyperparathyroidism. Whereas the normal parathyroid tissue floated, the diseased tissue invariably sank in a mannitol solution with a density range between 1.049 and 1.069. With the aid of this test, a diagnosis of adenoma was made in 66 patients and of primary hyperplasia in the remaining seven. In every case the diagnosis was subsequently confirmed. Forty-two of the 66 patients with an adenoma (64%) had a unilateral exploration of the neck. The Density Test saved these patients from an unnecessary contralateral exploration by the finding of an adenoma and a normal second gland in the same side of the neck. These data show that the Density Test is useful in the intraoperative diagnosis of a diseased from a normal parathyroid tissue. Tissue that sinks within the density range of 1.049 and 1.069 is without exception diseased and should therefore be either partially or completely excised even if the gland is of average size or only of slight enlargement. If it does not sink, it is virtually certain to be normal and should be spared. The Density Test provides a valuable clue in the differentiation of primary parathyroid hyperplasia from neoplasia.