We report a 29-year-old man with a mild decrease in glomerular filtration, nephrocalcinosis, hypercalciuria and a renal magnesium leak. He had other features of 'congenital magnesium-losing kidney', such as arthritis and hyperuricemia, short stature and recurrent urinary tract infections, but had no radiological chondrocalcinosis. In addition, pallidal calcification was found. The patient also had a renal phosphate leak. Phosphorus supplements resulted in a decrease in urinary calcium excretion, indicating that hypercalciuria was at least partially a consequence of phosphorus depletion. Plasma and urine magnesium were not affected by phosphorus supplements. Addition of magnesium supplements resulted in a transient and modest decrease in urinary calcium excretion, with no modification in plasma magnesium.