Background Intrathoracic impedance monitoring has emerged as a promising new technique for the detection of impending heart failure (HF). Although false positive episodes have been reported in case reports and clinical trials, the efficacy and false positive rate in real-world practice remain unclear. Objective The aim of this study is to investigate the utility and reliability of the OptiVol alert feature in clinical practice. Methods We continuously recruited patients who underwent implantable cardioverter-defibrillator (ICD) or cardiac re-synchronization therapy with defibrillator (CRT-D) implantation with feature of intrathoracic impedance monitoring system in our center from Sep. 2010 to Oct. 2012. Regular in-office follow-up were required of all patients and the following information was collected at each visit: medical history, device interrogation, N-terminal pro-brain natriuretic peptide (NT-proBNP) measurement and an echocardiogram. Worsening HF was defined as hospitalization or the presentation of signs or symptoms of HF. Results Forty three patients (male:76.7%, mean age:57 ± 15 years, left ventricular ejection fraction (LVEF):33%± 14%) were included in this observational study. Fifty four alert events and 14 adjudicated worsening HF were detected within 288 ±163 days follow-up. Eleven (20.4%) alert episodes were associated with acute cardiac decompensation in 9 patients with a positive predictive value of 78.6%. Forty three audible alerts showed no connection to worsening HF. The unexplained alerts rate was 79.6%and 1.27 per person-year. Thirty seven alarm alerts were detected in patients with EF<45%, among which 9 accompanied with HF, 17 alerts detected in patients with LVEF≥45%and 2 associated with HF. There was no sig-nificant difference between the two groups (9/37 vs. 2/17;P=0.47). Conclusions Patients with normal or nearly normal left ventricular systolic function also exhibited considerable alert events. The OptiVol fluid index predicted worsening cardiac events with a high unex-plained detection rate, and any alert must therefore be analyzed with great caution. Efforts to improve the specificity of this monitoring sys-tem represent a significant aspect of future studies.