The serum chloride and phosphate levels were measured and the chloride/phosphate ratios calculated in a group of eighty-four hypercalcemic patients. Although patients with hyperparathyroidism frequently had phosphate levels in the low normal range (less than 3 mg/100 ml) and chloride levels in the nigh normal range (greater than 102 mEq/L), they were nevertheless significantly different from the groups of patients with nonparathyroid hypercalcemia in whom phosphate levels were usually higher (greater than 3 mg/100 ml) and chloride levels usually lower (less than 102 mEq/L). The chloride/phosphate ratio was higher than 33 in 94 per cent of hyperparathyroid patients and lower than 33 in 96 per cent of other hypercalcemic patients. Thus, the measurements of serum phosphate and chloride levels and the calculation of the chloride/phosphate ratios were useful diagnostic screening tests that discriminated between patients with hypercalcemia of parathyroid and nonparathyroid origin with an accuracy of 95 per cent.