The relative importance of dietary factors in causing hypercalciuria was assessed in 282 unselected patients with calcium oxalate kidney stones. The 124 patients found to be hypercalciuric on either their customary free diet or on a 25-mmol (1000-mg) calcium defined diet (or both), were classified according to their pattern of calcium excretion on the two diets. Unexpectedly, about half of the patients who were hypercalciuric on their free diet exhibited a calcium excretion that fell markedly or normalized on the high-calcium defined diet. These patients were defined as having dietary hypercalciuria. For all 282 patients, multiple-regression analysis suggested that dietary sodium was at least as important as was dietary calcium, and more important than dietary protein, carbohydrate, phosphorus, purine, or oxalate, in contributing to calcium excretion on the free diet. Among the 124 hypercalciuric patients, urinary calcium excretion increased by 0.0193 mmol (0.77 mg) per mmol sodium excretion. Dietary habits, particularly a high sodium intake, may commonly contribute to hypercalciuria in patients with calcium oxalate stones.