At the age of 13 years, after 2 years of illness, a girl was hospitalized for abdominal pain, vomiting, weakness, and lethargy. A clue to the diagnosis was the finding of hypercalcemia (21.8 mg. per 100 ml. of serum). Surgical removal of a parathyroid adenoma from the right lobe of the thyroid was followed by prompt clinical improvement. Physical recovery was accompanied by a remarkable change in the personality of the child, who became bright and alert, thereafter did extremely well in school, and resumed normal social activity.