Both amount and timing of dietary calcium intake influence the recurrence of renal calcium stones. We have evaluated whether the hardness of extra meal drinking water modifies the risk for calcium stones. The urinary levels of calcium, oxalate and citrate, i.e., the main urinary risk factors for calcium stones, were measured in 18 patients with idiopathic nephrolithiasis, maintained at fixed dietary intake of calcium (800 mg/day), after drinking for 1 week 2 liters per day, between meals, of tap water and at the end of 1 week of the same amount of bottled hard (Ca2+ 255 mg/l) or soft (Ca2+ 22 mg/l, Fiuggi water) water, in a double-blind randomized, crossover fashion. As compared with both tap and soft water, hard water was associated with a significant 50% increase of the urinary calcium concentration in the absence of changes of oxalate excretion; the calcium-citrate index revealed a significant threefold increase during ingestion of hard water as compared with respect to soft water (Fiuggi water), making the latter preferable even when compared with tap water. This study suggests that, in the preventive approach to calcium nephrolithiasis, the extra meal intake of soft water is preferable to hard water, since it is associated with a lower risk for recurrence of calcium stones.