Some mineral waters have a high calcium content and may contribute a significant part of the human daily requirements. Calcium bioavailability from a calcium- and sulfate-rich mineral water (CS-W) containing 11.2 mmol Ca/L (467 mg/L) was compared with that from milk in nine healthy young women. Calcium absorption was measured in the fasting state with a dual-label stable-isotope technique. Fractional absorption rates from milk and CS-W were 25.0 +/- 6.7% and 23.8 +/- 4.8% (means +/- SD), respectively, and did not differ significantly (P = 0.05). Urine was collected for 36 h after the administration of the oral stable isotope while the subjects consumed a controlled diet and a quantity of milk or CS-W providing 25 mmol (1000 mg) Ca. No significant difference was found in the excretion of calcium, nor in the excretion of the two stable isotopes. Mean urinary sulfate excretion was significantly increased by 35% when the CS-W was consumed. No significant correlation was found between 36-h urinary excretion of the intravenous calcium tracer and sulfate, sodium, or urine volume. Therefore, calcium from the CS-W was as well absorbed and retained as that from milk, and no calciuric effect of sulfate was found, showing that such mineral waters can be valuable dietary sources of calcium.