1. Vitamin D seems to play an essential role in the pathogenesis of idiopathic hypercalciuria at least in part via intestinal hyperabsorption of calcium. Hyperabsorption of calcium, in turn, might enhance the intestinal uptake of free oxalate, thus leading to hyperoxaluria. To verify this hypothesis we studied 75 calcium-stone-formers subdivided as follows: group 1 (15 patients) with isolated hyperoxaluria; group 2 (25 patients) with hyperoxaluria and hypercalciuria; group 3 (22 patients) with isolated hypercalciuria; group 4 (12 patients) with no metabolic abnormalities. 2. As expected, urinary calcium excretion differed in the various groups (P < 0.001), being highest in groups 2 and 3; urinary oxalate excretion, by definition highest in groups 1 and 2, was even more pronounced in group 2 than in group 1 (P < 0.05). Although in the normal range, the serum 1,25-dihydroxyvitamin D concentration was higher (P < 0.001) in the two hypercalciuric groups (2 and 3), showing peak levels in group 2. 3. When the data from the 75 stone-formers were pooled, there was a positive correlation between the serum concentration of 1,25-dihydroxyvitamin D and urinary calcium excretion (P < 0.001) and urinary oxalate excretion (P < 0.003), the latter relationship also being present when only the two hypercalciuric groups (groups 2 and 3) were considered together (P < 0.05). 4. Our data seem to confirm a relevant role for the vitamin D system in the pathogenesis of calcium nephrolithiasis due to increased intestinal calcium absorption, but also because this in turn induces a greater intestinal absorption of oxalate, thus leading to the occurrence or exacerbation of hyperoxaluria.