Acute massive vitamin D overdosage occurred in a family after eating food cooked in a nut oil containing 5 million units of vitamin D3/ml. The plasma vitamin D was 55 and 60 i.u./ml in the father and mother respectively, and 9·6 i.u./ml in their 11-month-old infant (normal range, 0-1·6 i.u./ml). All the family presented with symptoms of hypercalcaemia and the infant responded quickly to prednisone. After steroids had failed to control the hypercalcaemia in the parents, neutral phosphate was successful, although necessary for 9 months. Before phosphate therapy it was shown that both parents were in strongly negative calcium balance, indicating that the vitamin D was mobilizing calcium from bone. Eleven years later all 3 patients are well but a renal biopsy in one of them shows persistent nephrocalcinosis.