The effect of prolonged (60 days) dietary phosphate restriction on divalent ion metabolism was studied in four patients with moderate renal insufficiency in an effort to delineate the mechanisms of secondary hyperparathyroidism in renal failure. The patients had low normal serum phosphorous and normal vitamin D metabolite levels but had evidence of disturbances in target organs for vitamin D, including impaired intestinal absorption of calcium, reduced calcemic response to PTH, low serum ionized calcium levels, and, consequently, elevated PTH levels. After dietary phosphate restriction, there was marked improvement or normalization of intestinal absorption of calcium, calcemic response to PTH, and ionized calcium and PTH levels. There was also a significant rise (44%) in serum 1,25-dihydroxyvitamin D levels. The data suggest that intracellular phosphorous retention, which may develop as renal insufficiency ensues, may interfere with the action and production of 1,25-dihydroxyvitamin D. This leads to defective intestinal calcium absorption and reduced calcemic responses to PTH. As a result, hypocalcemia develops, causing secondary hyperparathyroidism. Our data assign a critical role for a disturbance(s) in vitamin D metabolism in genesis of the hyperparathyroidism of renal failure.