A distinct advantage of implantable device diagnostics is that the data may be made available on a continuous basis, rather than at sporadic intervals associated with clinical testing. Recently, intrathoracic impedance monitoring has also become available in some implantable devices as an index of congestion and thoracic fluid accumulation secondary to decompensated heart failure. Despite the potential advantages, new advances in implantable device diagnostic technology also pose important questions regarding their clinical application. To examine these questions and to generate hypotheses, the relation between daily intrathoracic impedance measurements and other physical measurements or comorbidities was studied. A retrospective review was performed of 25 patients who previously underwent implantation of a cardiac resynchronization therapy/defibrillator device with the capability to continuously monitor intrathoracic impedance. This limited scope analysis demonstrated that daily measurement of intrathoracic impedances might reveal the intrinsic relations between heart failure decompensation and the onset of atrial and ventricular arrhythmias. Abnormal patterns of intrathoracic impedance that has increased and plateaued after implant may indicate worsening heart failure. The severity of congestive heart failure at the time of interrogation may correlate with device-based impedance measurements. Weight, sex, and body index may have a limited impact on impedance, and the lowest impedances may be seen in older patients. Patients with severe pulmonary disease may present with unique daily impedance profiles. Finally, daily impedance may have unpredictable relations with other clinical markers of heart failure. In summary, intrathoracic impedance represents a clinically useful diagnostic tool that can increase our understanding of a dynamic disease state on an individual patient basis.