Background Fluid overload is a risk factor for morbidity, mortality, and prolonged ventilation time after surgery. Patients on maintenance hemodialysis might be at higher risk. We hypothesized that fluid accumulation would be directly associated with extended ventilation time in patients on hemodialysis, as compared to patients with chronic kidney disease not on dialysis (CKD3-4) and patients with normal renal function (reference group).Methods this is a prospective observational study that included consecutive patients submitted to an isolated and elective coronary artery bypass surgery classified as reference (N=167, normal renal function), CKD3-4 (N=84, estimated glomerular filtration rate 30-60ml/min/1.73m 2 ), and hemodialysis (N=31, maintenance hemodialysis) groups. The same observer followed patients daily from the surgery to the hospital discharge.Results Fluid accumulation correlated with time on mechanical ventilation in patients on hemodialysis (r=0.627, p=0.003), but not in the CKD3-4 group (r=-0.068, p=0.652) and in the reference group (r=-0.085, p=0.363). Multivariate analysis revealed that the fluid accumulation, scores of sequential organ failure assessment-SOFA in the day following surgery, and the group according to renal function were independently associated with ventilation time. Furthermore, in patients on hemodialysis, the time between surgery and the first hemodialysis also accounted for the time on mechanical ventilation.Conclusions Fluid accumulation is an important risk factor for lengthening mechanical ventilation, particularly in patients on hemodialysis. Future studies are warranted to address the ideal timing for initiating dialysis in this scenario in an attempt to reduce fluid accumulation and avoid prolonged ventilation time and hospital stay.