Serum 25-hydroxycholecalciferol (25-OH-D3) was determined in 20 normal persons and in 50 patients with renal failure. Low serum levels were found in patients with markedly impaired renal function while patients on chronic hemodialysis exhibited normal values. Since uremia is assumed not to influence the resorption and hepatic conversion of vitamin D the decreased 25-OH-D3 levels are discussed to be due to low vitamin D intake on protein-restricted diet.