A female patient aged 45 years is described with a rare form of chondrocalcinosis caused by hypomagnesaemia due to excessive renal loss of magnesium. The patient also had impaired renal conservation of potassium leading to a hypokalemia. She most probably had an idiopathic renal tubular dysfunction. Magnesium supplementation prevented further symptoms. Therefore in young patients with chondrocalcinosis it can be of therapeutic importance to search for an underlying treatable metabolic disorder.