New onset diabetes mellitus (NODM) affects kidney transplantation outcome. Several risk factors, including immunosuppressive drug levels, are related with NODM development. This analysis evaluates the incidence and risk factors of NODM in kidney transplant patients receiving tacrolimus, taking into account 6-month blood levels and concentration-dose ratios (CDRs). Seventy-six patients under tacrolimus therapy who received a cadaveric renal transplant in our centre and with graft survival higher than 1 year were included in the study. NODM was defined as two fasting plasma glucose values > or =126 mg/dl or symptoms of diabetes plus casual plasma glucose concentrations > or =200 mg/dl throughout the first year. We examined previously reported variables related with NODM development. The incidence of NODM at 12 months was 27.6%. Risk factors for NODM included older age, higher first tacrolimus level, higher body mass index and lower first year weight gain. In multivariate analysis, the first year occurrence of NODM was significantly determined by the first tacrolimus blood level >20 ng/ml and age older than 50 years. CDR remains significantly higher in NODM throughout the 6 months. Older age and a high first tacrolimus blood level are associated with the development of NODM during the first year after kidney transplantation. NODM patients show higher CDR during the first 6 months.