The calcium content of the central one third of the skeleton was measured using neutron activation analysis in 109 patients with idiopathic calcium nephrolithiasis. The bone mineral content (calcium bone index or CaBI, corrected for body size) was significantly decreased by 5.2% in 20- to 60-year-old patients with calcium nephrolithiasis (p less than 0.01). Under age 50 the decrease was more marked in 64 males (7.1%; p less than 0.02) than in 21 females (4.1%; p = NS). There was a significant negative correlation of CaBI with fasting urine calcium/creatinine ratio (r = 0.39; p less than 0.01), but no correlation with age or indices of parathyroid function. The decrease in bone mineral content did not appear to be progressive. The decrease in CaBI indicates negative calcium balance, either in the past or at present, in patients with calcium nephrolithiasis and does not favour increased intestinal absorption as a primary cause. The lack of correlation of CaBI with parameters of parathyroid function does not support a primary renal loss of calcium. The results suggest that increased bone turnover may be an important component of disordered calcium metabolism in patients with calcium nephrolithiasis.