BACKGROUND Left ventricular diastolic dysfunction is common in hemodialysis patients and is associated with worse outcome. Previous studies have shown that diastolic function worsens from pre- to post-dialysis session, but this has not been studied during hemodialysis. We studied the evolution of diastolic function parameters early and late during hemodialysis. METHODS Observational study. METHODS 109 hemodialysis patients on a thrice-weekly dialysis schedule with a mean age of 62.5 ± 15.6 (SD) years were studied between March 2009 and March 2010. METHODS Hemodialysis with constant ultrafiltration rate and dialysate conductivity. RESULTS Changes in diastolic function parameters. METHODS Mitral early inflow (E) and tissue Doppler early diastolic velocity (mean e') were evaluated by echocardiography predialysis, at 60 and 180 minutes intradialysis, and postdialysis. Relative blood volume changes were calculated from changes in hematocrit. RESULTS Predialysis E and mean e' were 0.93 ± 0.24 m/s and 6.6 ± 2.1 cm/s, respectively. E and mean e' values decreased significantly during hemodialysis (P < 0.001). The steepest change occurred at 60 minutes intradialysis (E, -21.4% ± 17.6% and -30.5% ± 19.2% at 60 and 180 minutes, respectively; mean e', -16.0% ± 18.6% and -19.5% ± 21.8% at 60 and 180 minutes, respectively). At 60 minutes intradialysis, changes in relative blood volume and brain natriuretic peptide level were associated significantly with the change in E but not with the change in mean e'. CONCLUSIONS Changes in relative blood volume may not fully reflect central blood volume changes and do not capture the effect of blood loss to the extracorporal circuit. Left atrial volume was not measured. CONCLUSIONS Left ventricular diastolic function worsens early during a hemodialysis session. The decrease in mean e' at 60 minutes intradialysis was unrelated to changes in relative blood volume. Although this finding does not exclude a role of hypovolemia because of the limitations of the measurement of relative blood volume, it raises the possibility that non-volume-related mechanisms are involved in the early decrease in mean e' during hemodialysis.