<i>Background:</i> Although probing dry-weight improves blood pressure control, its effect on echocardiographic left ventricular mass index (LVMI) is unknown. <i>Methods:</i> Shortly following dialysis, 292 echocardiograms in 150 patients participating in the DRIP trial were obtained at baseline and longitudinally every 4 weeks on 2 occasions. <i>Results:</i> At baseline, LVMI was 136.3 g/m² in the control group and 138.7 g/m² in the ultrafiltration group (p > 0.2 for difference). The change from baseline in LVMI in the control group was +3.5 g/m² at 4 weeks and +0.3 g/m² at 8 weeks (p > 0.2 for both changes). The change from baseline in LVMI in the ultrafiltration group was –7.4 g/m² at 4 weeks (p = 0.005) and –6.3 g/m² at 8 weeks (p = 0.045). With ultrafiltration, the change in LVMI diameter was –10.9 g/m² more compared to the control group at 4 weeks (p = 0.012) and –6.6 g/m² more compared to the control group at 8 weeks (p = 0.21). The reduction in interdialytic ambulatory blood pressure was also greater in response to probing dry-weight in those in the top half of LVMI at baseline (p = 0.02 for interaction effect at week 8). <i>Conclusion:</i> LVMI, an important determinant of prognosis among long-term dialysis patients, is responsive to probing dry-weight.