Just as a stable defibrillation threshold is required for implantable defibrillators to maintain efficacy and a margin of safety for the conversion of life-threatening ventricular arrhythmias, a stable pacing threshold is also required to provide bradycardia support and pacing to terminate ventricular tachycardias. This article reports the temporal course of pacing thresholds in patients treated with a tripolar, tined endocardial defibrillator lead capable of bipolar sensing and pacing, and defibrillation. Seventeen patients who underwent implantation of an implantable defibrillator system using an integrated bipolar pacing/sensing system were prospectively studied over 18 months. There were 16 males and one female, with a mean age of 69 ± 5 years (range 61-75 years). At implantation, predischarge, and every 2 months thereafter, the pacing pulse-width threshold was tested at both 2.5 and 5.0 V stimulus amplitudes. After a mean follow-up of 363 ± 173 days (range 34-597 days), the pacing threshold increased from 0.08 ± 0.08 ms to 0.5 ± 0.3 ms at the 2.5 V amplitude (p ≤ 0.01, CI-0.57 to -0.27) and from 0.04 ± 0.02 ms to 0.25 ± 0.14 ms at the 5.0 V amplitude (p ≤ 0.01, CI -0.28 to -0.14). Eight of the 17 patients (47%) received spontaneous implantable defibrillator shocks for clinically detected arrhythmias, and the total number of joules delivered via the leads did not correlate with the pacing threshold changes. We conclude that the pacing threshold for the nonthoracotomy implantable defibrillator lead system studied is not stable and increases with time. This finding has implications for defibrillator battery life in patients who use implantable defibrillators for bradycardia pacing.