Hyperoxaluria is a disorder — usually inherited — in which the body produces excessive oxalic acid (oxalate), which crystallises with calcium to form kidney stones and, in severe types, deposits throughout the body. The primary forms are enzyme defects of liver metabolism (the commonest, Type 1, is alanine-glyoxylate aminotransferase deficiency); secondary hyperoxaluria follows high-oxalate diets, vitamin C megadoses, or fat malabsorption that lets more oxalate absorb from the gut. Sufferers pass recurrent, agonising calcium-oxalate stones, and untreated primary disease destroys the kidneys in childhood or youth. Management means massive fluid intake, citrate therapy, low-oxalate diet, and — now — RNA-interference drugs and liver transplantation for severe cases.