In this study, we determined the prevalence of idiopathic hypercalciuria (IH) in patients with renal lithiasis, and measured the bone mineral density (BMD) and biochemical markers of bone metabolism in patients with IH. Among 85 consecutive patients with urolithiasis (40 men, 30 postmenopausal and 15 premenopausal women), hypercalciuria (urinary calcium excretion >4 mg/kg per day) was observed in 22 (11 men, 8 postmenopausal and 3 premenopausal women). These 22 patients were then classified as having absorptive or fasting hypercalciuria. In 19 of the 22 hypercalciuric patients (I I men and 8 postmenopausal women), BMD was measured by dual-energy x-ray absorptiometry and serum levels of calcium, alkaline phosphatase, parathyroid hormone, osteocalcin (OC), and urinary deoxypyridinoline (DPD) were determined. When compared with age- and sex-matched control subjects (volunteers from hospital personnel), OC and urinary DPD levels were significantly higher in the renal lithiasis patients when compared with control subjects. Ward's triangle BMD in women and lumbar BMD in men were significantly lower when compared with sex- and age-matched control subjects. Lumbar, Ward's triangle, and femoral neck BMD measurements were inversely correlated with the duration of renal lithiasis in male patients. Males also had a significant negative correlation between lumbar BMD and the urinary DPD. BMD decreases significantly in male and postmenopausal female patients with IH. Patients with renal lithiasis should be evaluated for IH, and those with IH should be screened for osteoporosis. Studies determining the clinical and lifestyle consequences of IH and its related osteoporosis should be performed.