Left ventricular (LV) remodeling is defined as ''the genomic expression resulting in molecular, cellular, and interstitial changes that are manifested clinically as changes in size, shape, and function of the heart after cardiac injury" (1). In ESRD, LV remodeling manifests features of both concentric and eccentric LV hypertrophy, resulting from enlarged LV volumes and increased LV wall thickness (2). As we discuss here, LV mass can change because of a change in LV wall thickness, LV cavity size, or both. Increased LV mass, increased LV end-diastolic volume (LVEDV), and changes in LV geometry (LV mass/LVEDV) are all associated with poor prognosis in these patients (3,4). The Frequent Hemodialysis Network (FHN) trial is the first large-scale randomized controlled trial of frequent daily and nocturnal dialysis versus conventional dialysis (5). FHN investigators previously demonstrated that, compared with conventional hemodialysis, frequent daily hemodialysis results in a significantly greater reduction in LV mass, as determined by cardiac magnetic resonance imaging (MRI) (6). In this issue of CJASN, this group reports that frequent daily dialysis resulted in greater reductions in LV and right ventricular (RV) volumes without significant changes in LV geometry (7). In addition, reductions in both LV mass and LVEDV with frequent dialysis were significantly correlated with reductions in predialysis systolic BP (6,7). Here we present a brief overview of the literature on the effects of dialysis on LV remodeling, discuss likely mechanisms underlying the findings of the current study, and reflect on implications for future investigation.Observational studies of frequent dialysis have shown salutary effects on BP (8-11) and LV mass index (12-15) and impressive survival rates (8,16). Moreover, LV hypertrophy has regressed in patients with ESRD who have baseline LV hypertrophy when aggressive management of BP and anemia was added to a thrice-weekly dialysis regimen (17,18). One such study shows that LV hypertrophy may regress without a change in relative wall thickness, another measure of LV geometry (18). In several studies, including the FHN trial, lowering BP was strongly associated with regression of LV hypertrophy (9,10,(12)(13)(14). Prior studies of frequent dialysis that evaluated LV size in addition to mass did not specifically address LV geometry (10,13).LV mass and volume are linked mathematically, physiologically, and empirically. Cardiac MRI is widely considered to be the gold standard for determining LV volume and mass, while M-mode echocardiography overestimates LV mass in patients with ESRD undergoing hemodialysis because of its geometric assumptions (19). In cardiac MRI, LV mass is calculated as myocardial volume multiplied by the specific density of the myocardium (20). Myocardial volume, in turn, is the difference between the enddiastolic epicardial volume and end-diastolic endocardial volume, excluding the papillary muscles (LVEDV). Thus, while MRI-determined LV mass is free of geometric assumptions, it ...