Intrathoracic impedance measurement has been introduced in the InSync Sentry biventricular implantable cardioverter-defibrillator and may permit the early identification of pulmonary fluid accumulation secondary to left-sided heart failure (HF). An audible alarm (the OptiVol alert) can be triggered when the impedance index increases to greater than a predefined level of 60 Ω · day. The aim of this study was to evaluate the clinical value of the OptiVol alert and its prediction for decompensated HF. One hundred fifteen consecutive patients (mean New York Heart Association class 2.8 ± 0.5, mean left ventricular ejection fraction 26 ± 8%) who received InSync Sentry biventricular implantable cardioverter-defibrillators were included. When presenting with the OptiVol alert, current hemodynamic status was evaluated. During follow-up (mean 9 ± 5 months), there were 45 presentations with the OptiVol alert in 30 patients. Clinical signs and symptoms of HF were present in only 15 patients (33%), whereas in the remaining patients, clinical signs of HF were absent (p <0.05). Receiver-operating characteristic curve analysis showed that increasing the threshold for the OptiVol alert provided a substantial increase in specificity for the detection of HF, with the optimal cut-off value identified at 120 Ω · day, yielding sensitivity of 60% and specificity of 73%. In conclusion, intrathoracic impedance measurement as present in the InSync Sentry biventricular implantable cardioverter-defibrillator may be a useful tool for monitoring pulmonary fluid status. The proposed threshold for the OptiVol alert of 60 Ω · day is very sensitive but not specific for the assessment of HF; adjustment of threshold settings may yield a superior balance between sensitivity and specificity.