As urinary prostaglandin excretion might be involved in idiopathic hypercalciuria, we studied the excretion of prostaglandin E2 and calcium together with serum prostaglandin E2, parathormone and 1,25 dihydroxyvitamin D in 20 patients and 11 controls Idiopathic hypercalciuric patients showed increased levels of urinary prostaglandin E2-like activity, which is correlated with calcium excretion. Although no change was observed in serum parathormone level in control and hypercalciuric patients, there was a positive correlation between serum 1,25 dihydroxyvitamin D and prostaglandin E2-like activity. These findings suggest that prostaglandin E2 could play a role in the hypercalciuria syndrome, possibly by increasing 1,25 dihydroxyvitamin D synthesis.