Objective. To know the risk factors for posttransplant diabetes mellitus (PTDM) and its effects on biochemical parameters and graft function at the end of 5 years. Materials and Methods. We retrospectively analyzed the 218 records of postrenal transplant patients who had a minimum follow-up for 5 years. Patients were divided into those with diabetes mellitus (DM; n = 21), PTDM (n = 58) and non-DM-non-PTDM (n = 139). Results. 29.4% of non-diabetics developed diabetes mellitus after transplantation in 5 years, with cumulative rates of 14.01% and 19.8% at 3 and 12 months, respectively. The incremental incidence was 14.97% during the first posttransplant year. The earliest presentation of PTDM was 9 days after transplant, and 53.45% of patients were asymptomatic at presentation. The following were risk factors: recipient age >36 years, hepatitis C virus infection, HLA-B13, family history of DM, body mass index >30, and calcineurin inhibitor therapy. These features were not risk factors: donor age, donor sex, recipient sex, cadaver donor, and antirejection therapy. The PTDM group members received the same number of antihypertensive drugs and statin doses and displayed similar levels of proteinuria. PTDM had no influence on biochemical parameters. The PTDM group had reduced graft function compared with non-DM non-PTDM subjects, when used glomerular filtration rate (estimated by renogram and calculated by MDRD formula) as marker, but not creatinine. The rate of urinary tract infections was higher among the PTDM group. Conclusion. Regular screening of plasma glucose is recommended from the early transplant period, particularly among high-risk patients. Regular monitoring of graft function using the MDRD formula or isotope renogram is necessary as PTDM influences graft function.