The probability of being a stone former (PSF) was calculated in 3 groups of idiopathic calcium stone formers [with normocalciuria (NC), dietary hypercalciuria (DH) and idiopathic hypercalciuria (IH)] in 4 conditions: while on a free diet; on a calcium- and oxalate-restricted diet during 4 days; after an oxalate load, while on a 1.5-gram calcium diet, and after an oxalate load while on a calcium-restricted diet. Combined calcium and oxalate restriction significantly decreased PSF only in NC and DH whereas the decrease was not significant in IH because of a concomitant significant increase in oxalate excretion. Increase of PSF with the oxalate load was significantly greater during a calcium-restricted diet than during the 1.5-gram calcium diet in all groups of patients (4, 6 and 12 times greater in NC, DH and IH, respectively). These data show the critical role of oxalate restriction when calcium is restricted in order to decrease the PSF. This combined restriction is however not sufficient in idiopathic hypercalciuric patients to decrease their PSF.