A group of 12 patients with recurrent calcium urolithiasis followed a uniform dietary regimen involving a prescribed intake of plain water during the first week and Donat Mg, a natural mineral water, during the second week of the study. 24-hour urine specimens were collected on the 6th and 7th days of each week. The analyses included measurement of the volume, relative density, osmolality and pH, and determination of the excreted quantity of magnesium, calcium, oxalate, urate, citrate, phosphate and creatinine. The two sets of values obtained for each patient were compared and significance of the differences was assessed using Student's t-test. The changes in urinary composition observed after the patients drank Donat Mg were in accordance with the theoretical expectations. The results showed a significant increase in the excretion of magnesium (p < 0.001) and citrate (p < 0.005), known to inhibit calculi formation, a shift in urinary pH towards the neutral range and a significant decrease in values of the risk indices Ca/Mg (p < 0.005) and oxalate/citrate (p < 0.001).