Because factual basic knowledge is lacking regarding the natural history of primary hyperparathyroidism, it has been impossible to make a reasonable prognostication of the risks inherent to judicious observation as a justifiable alternative to neck exploration in patients with mild asymptomatic and uncomplicated hyperparathyroidism. In Jan. 1968 a prospective study was initiated at the Mayo Clinic [Minnesota, USA] to provide more information regarding the natural history of this disease. The cumulative 10 yr findings of this study are presented. It was not possible to define criteria that will predict which patients with primary asymptomatic hyperparathyroidism will ultimately require surgery.