Lead failure places patients with implantable cardioverter defibrillators (ICD) at risk for sudden cardiac death or results in delivery of inappropriate shocks. This study describes a mechanism of lead malfunction occurring at the junction of the terminal ring with the conductor coil of the rate sensing terminal connector in one specific model of a transvenous ICD lead. We detected the problem in a population of 179 patients with a mean age of 61 +/- 10 years and a mean lead implant duration of 16 +/- 11 months. All patients underwent pectoral ICD implantation using a submuscular approach. The implanting physician chose to place the ICD on the left side in 155 patients (87%) and on the right side in 24 patients (13%). Cephalic vein cutdown provided central venous access in 147 patients (82%), and subclavian vein puncture provided access in 32 patients (18%). Follow-up examination detected lead failure in six patients (3.5% over 31 months) due to insulation damage with or without conductor coil fracture at the junction of the terminal ring and conductor coil of the IS-1 rate sense terminal. We detected lead disruption 17 +/- 9 months (range 5-31 months) after implantation. Multiple nonsustained arrhythmia episodes exhibiting nonphysiologic intervals associated with noisy rate sensing electrograms during pocket manipulation led to discovery in three patients. The other three patients presented with inappropriate device discharges confirmed by stored high-energy lead electrograms showing normal rhythm. Pacing lead impedance abnormally dropped in two patients. Impedance remained stable in the other four patients. In conclusion, the generator pocket represents an important site of ICD transvenous lead vulnerability. Lead failure may result from conductor coil and/or insulation disruption at the interface with the rate sensing terminal connector.