The authors report the cases of 13 patients with parathyroid adenomas and hyperparathyroidism. They estimate that this disease may be observed in about 70/100,000 patients hospitalised in a non-specialised medical department. In four of these patients, the disease was asymptomatic while only four patients presented with symptomatic renal stones. In five patients, immuno reactive parathyroid hormone levels were within normal limits. In contrast, nephrogenic cyclic AMP, measured in 11 patients, was found to be increased in the totality of these patients and could not be decreased by the administration of calcium. An ultrasonographic examination of the neck, performed in 11 patients, showed the adenomas in 6 patients; in three other patients they were located in the neck but measured less than 1 cm in their greater diameter and in other two patients the adenomas were located in the thymus.