A 39-year-old man with a lifelong history of tetany and hypocalcaemia was found to have hypomagnesaemia (0·29 mmol/l) due to renal magnesium loss. His asymptomatic 29-year-old brother had a similar disorder. Both were infertile and had severe oilgospermia but normal endocrine function. They had medullary nephrocalcinosis and glomerular filtration rate was reduced. Renal biopsy showed patchy interstitial fibrosis and some glomerular sclerosis. Electron microscopy showed thickened basement membranes in damaged glomeruli and in tubules in areas of fibrosis. Tests of renal tubule function were normal. Hypocalcaemia and tetany were corrected by oral magnesium supplements which raised the serum magnesium level to around 0.54 mmol/l.