In 22 patients who underwent surgery suspected of primary hyperparathyroidism, the surgical findings were compared with the results obtained by pre-operative parathyroid scanning and biochemical screening. Thirteen of 15 parathyroid adenomas were localized by pre-operative scanning, but in five of them a false positive focus was also described. The technique was less useful in primary hyperplasia. Comparable results were reported by other investigators. In both instances the best results were obtained in patients with high parathyroid activity as measured by plasma calcium, plasma alkaline phosphatase and tubular reabsorption of phosphorus (TRP). Parathyroid scintigraphy was especially helpful in the presence of ectopic adenomas and in patients who had undergone previous parathyroid surgery. Unfortunately, the possibility of false positive results makes it unreliable for the diagnosis of primary hyperparathyroidism.