The serum levels of 25-hydroxycholecalciferol (25-OHD3) and 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] were measured simultaneously in nephrectomized patients on maintenance haemodialysis, in haemodialyzed patients with preserved kidneys who were receiving different vitamin D supplement, and in patients who had undergone renal transplantation. The results indicate that the production of 1,25-(OH)2D3 can be stimulated in patients with minimal residual renal excretory function by increasing the serum levels of 25-OHD3. Successful renal transplantation was followed by a rise in serum 1,25-(OH)2D3 concentrations.