The files were studied of 300 patients operated for primary hyperparathyroidism for the first time. Their median age was 60 years. The female/male ratio was 3/1, but in the younger patients males and females were about equally present. Preoperatively, one third was considered as asymptomatic with respect to hyperparathyroidism. In this subgroup the hypercalcemia was detected coincidentally and the mean parathyroid hormone level was lower than in the others. Another third of the patients had nephrolithiasis, they were on the average younger and there were more males. Finally one third showed other symptoms as gastrointestinal disease, bone disease or general malaise. Intraoperatively, we found a solitary adenoma in 90% of the cases, a double adenoma in 5% (on each side of the neck in half of the cases) and hyperplasia in 4%. The adenomas had a tendency to occur more often in the upper parathyroid glands, but the difference was not important enough to influence the surgical technique. Hyperplasia patients were, on the average, younger and double adenoma patients older. The female/male ratio was 1/1 for hyperplasia and 15/1 for double adenoma.