Surgical exploration of the parathyroid glands was carried out in 84 patients who had recurrent kidney stones and serum calcium levels in the upper quartile and most of whom had hypercalciuria. Parathyroid adenoma(s) were found in 19 cases, hyperplasia in 39 cases, and normal parathyroid glands in 26 cases. Postoperatively, a clinical follow-up was carried out for 2 to 5 years. No relapse has occurred in the cases with adenoma(s) but did occur in 24% of the group with hyperplasia and in 48% of the group with normal glands. The histopathologic findings are described here, while the clinical results are given in another paper. The adenomas do not differ histologically from those giving rise to hypercalcemic hyperparathyroidism. The hyperplasia was of the chief cell type and was slight in most cases. The "normal" glands did not differ from other normal glands from euparathyroid subjects. There was no significant difference in weight and histopathologic appearance between the hyperplastic glands of patients who relapsed and those who did not. Nor did the normal glands of "cured" patients differ from those of patients with relapse. However, in both these groups, some histologic features seem to indicate a favorable outcome; in the group with hyperplasia, there were higher glandular and parencymal cell weight as well as predominance of light chief cells and small fibrotic areas. In the normal group, higher number of argyrophil cells and small fibrotic areas also seem to implicate a better prognosis.