Magnesium deficiency can develop in patients with acute or chronic liver disease as a result of low dietary magnesium intake, low intestinal absorption or renal magnesium loss caused by natriuretic drugs. The aim of this study was to evaluate magnesium homeostasis in 39 children: 10 with acute liver failure due to Amanita phalloides poisoning. 14 with chronic liver diseases without cholestasis, and 15 with chronic liver diseases with cholestasis. Serum magnesium and fractional and 24 h urinary magnesium excretion were measured in all the children. Magnesium retention after intravenous infusion was also evaluated. Tissue magnesium deficit was found in 30 per cent of children with acute or chronic liver disease.