Objective: In this study, we aimed to identify patients at risk of chemotherapy-induced cardiotoxicity with a simple method like platelet-to-lymphocyte ratio (PLR) before starting therapy. Method: A total of 65 breast cancer patients who completed anthracycline or adjuvant trastuzumab treatment were evaluated retrospectively. Serial PLR calculations, echocardiographic examinations, and cardiac markers before treatment and after follow-up period were analyzed. Cardiotoxicity was determined according to Cardiac Review and Evaluation Committee Criteria. Results: Patients were divided into two groups according to their baseline (...) PLR levels as Group C—PLR < 119 and Group D—PLR 120. The median follow-up of the study was 22.23 (12-42) months. Concomitant disease and baseline characteristics were similar in both groups. Symptomatic cardiotoxicity was not observed in both groups. Cardiotoxicity was occurred in one patient (2.3%) in Group C and in four patients (9.5%) in Group D (P ¼ .005). Average mean left ventricular ejection fraction loss from baseline was 10.7 6 7.0% in Group D vs 2.3 6 6.4% in Group C (P ¼ .008). Interpretation of cardiac markers that were present in nearly half of the patients revealed that serum hs-c-reactive protein and pro-brain natriu¨ retic peptide levels were significantly higher in patients who developed cardiotoxicity compared to who did not develop cardiotoxicity. PLR 120 had 99% sensitivity and 85% specificity in predicting cardiotoxicity. Conclusion: This study’s results showed that high PLR levels were associated with chemotherapy-induced cardiotoxicity. To our best knowledge, this is the first study, examining the impact of whole blood test on chemotherapy-induced cardiotoxicity before starting the therapy and allowing doctors plot a route for these risky patients. (shrink)