Posttransplant diabetes mellitus (PTDM) occurs at varying rates in renal transplant recipients. Although its exact cause is not known, it is known to have an association with male gender, black race, the use of immunosuppressants and with acute rejection episodes. In this study we have evaluated patients with PTDM in relation to early graft function, type of donor and rejection episodes. Twenty-six renal transplant recipients with PTDM (20 M, 6 F: 19 from living related and 7 from cadaveric donor) were included in this study. Early graft function was assessed by Tc-99m DTPA renal scintigraphy obtained on postoperative day 3 and 7. Scintigraphic evaluation was based on visual and quantitative evaluation of perfusion, concentration and excretion pattern of the grafted kidney. Pre and postoperative blood creatinine levels obtained within the first month and the number of acute rejection episodes within the first 2-year after the operation has been recorded. PTDM cases and episodes of acute rejection that occured within 3 week postoperation were regarded as early onset. The source of the graft kidney was a living related donor in 73% of the recipients. Sixteen patients (62%) become hyperglycemic within 3 weeks of posttransplantation. Twenty-two out of 26 patients (85%) had acute rejection episode (13 early and 9 late). All except 5 patients (81%) had a normal scintigraphy on day 7 after the operation. Three out of five patients with impaired graft function had early acute rejection. None had acute tubular necrosis during the early postoperative period. Our results have shown that the presence of PTDM is closely linked to normal early postoperative graft function, the presence of a living related donor and acute rejection episodes. Elevated blood viscosity is known to have an association with insulin resistance. We hypothesize that a rapid increase in blood viscosity as a result of excellent early graft function might be responsible for PTDM.