Urinary oxalate excretion was measured in 101 male idiopathic calcium (Ca) stone formers studied on 3 dietary conditions (free-choice, Ca-enriched, and low-Ca diet). The population consisted of 38 normocalciuric and 63 hypercalciuric patients. Mean oxalate excretion was similar in normocalciuric and in hypercalciuric patients, on free-choice as well as on Ca-enriched diet. In both conditions the incidence of hyperoxaluria (greater than or equal to 435 mumol/24 h) within each group of stone formers was also similar, ranging from 11 to 22%. On low-Ca diet, however, mean oxalate excretion increased significantly (p less than 0.01) in hypercalciurics but not in normocalciurics; on this diet, the incidence of hyperoxaluria was particularly high in the hypercalciurics (33%), compared with the normocalciurics (13%). On low-Ca diet, oxalate excretion was positively correlated with the estimated degree of intestinal absorption of calcium (p = 0.01). These results show that among idiopathic stone formers, mild hyperoxaluria is not a rare finding and that this disorder can be encountered in each group of patients; its incidence, however, is influenced by the calcium content of the diet. On a low-Ca diet, patients with intestinal Ca hyperabsorption are particularly prone to develop hyperoxaluria, an observation which leads to question the relevance of such a dietary advice unless oxalate intake is simultaneously reduced.