Background: Various factors associated with worsening heart failure (HF) events have been investigated in HF subjects. The purpose of this study was to identify the predictive factor(s) for worsening HF events after cardiac resynchronization therapy (CRT) among baseline parameters, as well as baseline factors associated with responsiveness or non-responsiveness to CRT. Methods and Results: Seventy-seven HF patients with an indication for CRT were enrolled. Baseline parameters of blood chemistry, electrocardiogram, echocardiogram and cardiac catheterization before device implantation were measured, and subsequent clinical HF events after CRT were investigated. During the follow-up period (median 601 days), 22 of 77 (29%) recipients had HF events (unscheduled HF hospitalization: 16; use of left ventricular assist system: 1; heart transplantation: 1; cardiac death: 4). In the multivariate Cox proportional hazards model, low serum sodium concentration was associated with the occurrence of HF events after CRT (hazard ratio 0.82, 95% confidence interval 0.68–0.99, P=0.034). At baseline, serum sodium concentration negatively correlated with pulmonary capillary wedge pressure (r=−0.71, P<0.001) and with plasma arginine vasopressin level (r=−0.68, P=0.008). Conclusions: Hyponatremia is an independent predictor for worsening HF events after CRT implantation, which may be partly explained by elevated level of plasma arginine vasopressin.  (Circ J 2013; 77: 116–122)