BACKGROUND: The relationship between body mass index (BMI) and patient survival in end-stage kidney disease is not well understood and has been the subject of much debate over recent years. METHODS: This study used a latent class joint modeling approach to identify latent groups that underpinned associations between patterns of change in BMI during hemodialysis and two competing events: transplant and death without transplant. We included all adult patients who initiated chronic hemodialysis treatment in Australia or New Zealand between 2005 and 2014. RESULTS: There were 16,414 patients included in the analyses; 2,365 (14%) received a transplant, 5,639 (34%) died before transplant, and 8,410 (51%) were administratively censored. Our final model characterized patients based on five broad patterns of weight change (BMI trajectories): "late BMI decline" (about 2 years after commencing hemodialysis); "rapid BMI decline" (immediately after commencing hemodialysis); "stable and normal/overweight BMI"; "stable and morbidly obese BMI"; or "increasing BMI." Mortality rates were highest among classes with declining BMI, and the timing of weight loss coincided with the timing of increases in mortality. Within the two stable BMI classes, death rates were slightly lower among the morbidly obese. CONCLUSIONS: The findings from this descriptive analysis suggest a paradoxical association between obesity and better survival. However, they also suggest that the shape of the BMI trajectory is important, with stable BMI trajectories being beneficial. Future research should be aimed at understanding the causes of weight changes during dialysis, to determine whether there could be strategies to improve patient survival.