This study was performed to evaluate the antihypercalciuric effect of calcitonin (CT), a potent inhibitor of bone osteoclastic activity, on idiopathic hypercalciuria (IH). Forty-two stone formers were studied: 18 suffered from fasting hypercalciuria (FH), 12 from nonfasting hypercalciuria (NFH) and 12 were normocalciuric stone formers (NSF). All patients received CT, 25 U/day sc for a period of 15 days. CT caused a statistically significant drop in urine calcium, phosphorus and hydroxyproline (OH-proline) excretion in FH patients and a concomitant increase in serum PTH levels. In this group the percentage variation (D%) of urine calcium decrease was linearly correlated with D% decrease in urine OH-proline. These results support the hypothesis that pathological bone reabsorption might be involved in the genesis of FH.