We sought to determine if the high phosphorus, moderately low calcium intake typical of U.S. teenagers and young adults alters parathyroid function as it does in experimental animals. Because those animals ultimately developed osteopenia, it has been suggested that low dietary calcium to phosphorus ratios may reduce peak bone mass and increase susceptibility to osteoporotic fracture later in life. However, it is not known whether PTH secretion or action increases in response to commonly consumed phosphorus-rich, calcium-poor foods. We studied the 24-h mineral and hormonal responses of eight men and eight women, aged 18-25 yr, after 8 days of ingesting a control diet that had calcium (820 mg) and phosphorus (930 mg) contents near the recommended daily intakes, and a test diet with calcium and phosphorus contents (1660 mg phosphorus, 420 mg calcium) typical of current intakes. Both diets were made from common grocery store foods. The 24-h mean serum immunoreactive PTH levels increased in men (11%; P less than 0.006) and women (22%; P less than 0.003) during the test diet. In both sexes, the test diet significantly increased serum phosphorus, plasma 1,25-dihydroxyvitamin D, and urinary hydroxyproline and cAMP excretion; in women only it decreased serum ionized and total calcium levels. Thus, short term ingestion of a diet typifying current levels of calcium and phosphorus intake resulted in elevated serum iPTH levels and indexes of PTH action in young adults.