OBJECTIVES: To study the body mass index (BMI) trajectory in patients with incident end-stage kidney disease and its association with all-cause mortality. METHODS: This longitudinal cohort study included 17022 adult patients commencing hemodialysis [HD] (n = 10860) or peritoneal dialysis [PD] (n = 6162) between 2001 and 2008 and had >/=6-month follow-up and >/=2 weight measurements, using the Australia and New Zealand Dialysis and Transplant Registry data. The association of time-varying BMI with all-cause mortality was explored using multivariate Cox regression models. RESULTS: The median follow-up was 2.3 years. There was a non-linear change in the mean BMI (kg/m2) over time, with an initial decrease from 27.6 (95 confidence interval [CI]: 27.5, 27.7) to 26.7 (95 CI: 26.6, 26.9) at 3-month, followed by increments to 27.1 (95 CI: 27, 27.2) at 1-year and 27.2 (95 CI: 26.8, 27.1) at 3-year, and a gradual decrease subsequently. The BMI trajectory was significantly lower in HD patients who died than those who survived, although this pattern was not observed in PD patients. Compared to the reference time-varying BMI category of 25.1-28 kg/m2, the mortality risks of both HD and PD patients were greater in all categories of time-varying BMI 28.1 kg/m2 among HD patients, but only in the category 28.1-31 kg/m2 among PD patients. CONCLUSIONS: BMI changed over time in a non-linear fashion in incident dialysis patients. Time-varying measures of BMI were significantly associated with mortality risk in both HD and PD patients.