The possibility that low-calcium intake in renal stone formers could lead to reduced bone mineral content was investigated in 123 male patients with idiopathic urolithiasis. Radius bone mineral content (BMC) was measured by single photon absorptiometry. Two groups of patients were analyzed: group 1 (n = 63) maintained on a free diet; group 2 (n = 60) maintained on a low-calcium diet (350 mg/day +/- 20 SEM) for 3.9 years +/- 0.6 SEM. The two groups of patients were investigated after a standard reduction of calcium intake for at least 1 week. The urinary excretion of calcium and of hydroxyproline, and the serum alkaline phosphatase activity were higher in both groups than in normal subjects submitted to the same low-calcium diet. Both groups of stone formers showed lowered radius BMC values at 3 cm (distal) and 8 cm (proximal) above the styloid process, but distal BMC was significantly lower in group 2 than in group 1. The results suggest that low-calcium intake could worsen the already decreased BMC of idiopathic renal stone formers.