Diabetes mellitus appearing after kidney transplantation--posttransplant diabetes mellitus (PTDM)--is a common complication associated with poor graft and patient survival. The purpose of the current study was to determine the risk factors for developing PTDM in 204 renal transplant recipients who had been followed for at least 30 months. Posttransplant diabetes mellitus was diagnosed according to the American Diabetic Association/WHO criteria, or a requirement for insulin, an oral hypoglycemic agent, or both. Analyses of possible risk factors for PTDM included demographic features, dialysis and posttransplantation duration, smoking, body mass index, medications, co-morbid diseases, HLA mismatches, as well as laboratory metrics of serum creatinine, albumin, calcium, phosphorus, C-reactive protein, parathyroid hormone, and lipid profiles. Twenty-six patients displayed PTDM. Univariate analysis showed that older age, greater body mass index, presence of hepatitis C virus (HCV) infection, and smoking at the time of renal transplantation were associated with PTDM development. In a multivariate analysis, HCV infection, smoking, and patient age at the time of transplantation were independent risk factors for PTDM. In conclusion, the presence of HCV infection or a smoking habit in addition to older age at the time of transplantation were the main predictors for developing PTDM. Patients should be closely followed regarding their smoking habit and weight gain as modifiable risk factors for PTDM.