Type I diabetes patients with end stage renal failure are suitable candidates for simultaneous pancreas and kidney (SPK) transplantation. C-peptide is used to monitor graft function. This study aimed to look at biological variation and determine a reference interval for c-peptide in SPK patients. 347 c-peptide results from 26 patients were obtained from the laboratory database. The quoted reference interval for c-peptide for a healthy fasting individual with normal blood glucose, is 350-1800 pmol/L. For SPK patients a reference interval of 441 – 3068 pmol/L may be more appropriate.