New-onset diabetes after transplantation (NODAT) is a serious complication after kidney transplantation. Obesity was widely identified as a modifiable risk factor for NODAT. Body mass index (BMI) is the most frequently used diagnostic indication of obesity, and higher pretransplant BMI has been reported to be an independent risk factor of NODAT. However, the influence of posttransplant increase in BMI on the development of NODAT during outpatient follow-up has not been established. This is a single-centered retrospective study in Japan. We identified 158 consecutive patients who received living donor kidney transplantation in Sendai Shakaihoken Hospital from September 2000 to December 2009. Of these, 101 patients were included in this study. NODAT was defined based on the American Diabetes Association definitions. Fifteen patients developed NODAT with a median follow-up period of 27 (3-109) months. Of these 15 patients with NODAT, 13 patients were diagnosed after the first year of transplantation, with a median follow-up of 29 months, and 2 patients were diagnosed at 3 months after transplantation. Recipient age (HR: 1.06 [1.01-1.13]) and increase in BMI (HR: 1.12 [1.01-1.26]) proved to be independent risk factors of NODAT in multivariate logistic analysis after adjustments for pretransplant 2-hour OGTT level, pretransplant BMI, and use of tacrolimus. This is the first study showing the association between an increase in BMI and the development of NODAT. The increase in BMI might be a risk factor for NODAT. These findings underline the importance of routine BMI measurements in medical practice.