Abstract Background Transient variations in physiological parameters may forewarn of life-threatening cardiac events, but are difficult to identify clinically. Implantable cardioverter defibrillators (ICD) designed to measure transthoracic impedance provide a surrogate marker for pulmonary congestion. Objective The aim of this study is to determine if the frequency of changes in transthoracic impedance (TTI) is associated with congestive heart failure (CHF) exacerbation and predicts mortality. Methods We followed 109 consecutive patients (pts) with ICDs (n=58) or CRT-ICDs (n=51) for a mean of 21.3 (+10.2) months. Using 80ohm-days as a reference, we correlated the frequency of TTI changes above this index to CHF hospitalizations or death. Results There was at least one TTI threshold crossing in 79 (72%) pts over 23.3months follow-up, with a mean of 1.8±3.4 per year. There were 18 pts with CHF hospitalizations who had a mean of 4.3 TTI threshold crossings/year (S.D.=±7.3; median=2.8), compared to 1.3 (S.D.=±1.5; median=0.8) among pts without CHF hospitalizations (p=0.0006). Among 20 patients who died during follow-up, there were 4.2 (S.D.=±7.0; median=2.9) TTI threshold crossings/year, compared with 1.3 (S.D.=±1.3; median=0.9) threshold crossings/year among survivors (p=0.0004). Using Cox Proportional Hazard modeling, after adjusting for age, baseline EF, and number of shocks, TTI threshold crossing was an independent predictor of death (HR 1.72, 95% CI 1.26–2.36, p=0.001). Conclusions Increased frequency of TTI threshold crossings may be a useful predictor of transient risk for identifying a subgroup of ICD recipients at greater individual risk for death or CHF hospitalizations.