Mental disturbances are found in more than 50% of patients with hyperparathyroidism. The mental disturbances consist of slight or severe neurasthenic personality change, principally characterized by lack of initiative and depression, and acute organic psychosis in the most severe cases. The mental disturbances are wholly reversible when the serum calcium returns to normal after successful extirpation of the parathyroid adenoma. Experience has shown that the mental changes and serum calcium levels are correlated: the higher the serum calcium, the more severe is the mental disturbance.