Background: Platelet-to-lymphocyte ratio (PLR) and relative lymphocyte count (L%) are commonly available tests that can be obtained fromcomplete blood count. The aim of this study was to investigate the association between appropriate defibrillator therapy and PLR, and whether decreased lymphocyte count may predict appropriate implantable cardioverter defibrillator (ICD) shocks in heart failure (HF) patients. Methods: A total of 147 patients with ischemic or non-ischemic HF who underwent ICD implantation for primary prevention were enrolled in this study. Peripheral venous blood samples were drawn on the same day as ICD implantation.White blood cell counts with differentials, red blood cell indices, and platelet counts were calculated with an automated blood cell counter. All patients were evaluated according to the presence of appropriate ICD therapy. Results: Baseline ejection fraction was significantly lower in the appropriate shock received group (p = 0.040). Median PLR was significantly higher and L% was significantly lower in the appropriate shock received group (p < 0.001). In both ischemic and non-ischemic HF groups, median L% was significantly lower in the appropriate shock received group (p < 0.001; p = 0.006, respectively). In multivariable logistic regression analysis, only L% showed a strong association with appropriate shock therapy (p < 0.001). Conclusions: Higher PLRs are related to appropriate shocks in patients that received ICD with lower EF. Furthermore, decreased L% is independently associated with appropriate shocks in HF