We describe here the case of a 58-year-old female patient who experienced inappropriate shocks from her implantable cardioverter-defibrillator (ICD). Stored electrograms from her ICD showed high frequency noise preceding the shock. Although the pacing threshold was normal and lead fracture was not found in chest X-rays, pacing lead impedance decreased to 480 omega. Moreover, such high frequency noise was observed by electrogram telemetry, but not by routine evaluation every 3 months. ICD lead dysfunction was suspected, so we elected to replace the ICD lead system. At the time of the operation, lead impedance was 410 omega and pacing threshold was the same as it was at the time of the ICD implantation, and no lead insulation disturbances were observed in the generator pocket. However, manipulation of the lead system produced high frequency noise reproducibly. Since some of the ICD lead dysfunction initially was clinically silent at rest, dysfunction was difficult to detect before serious problems occurred. Therefore, more careful evaluation of the ICD lead system is needed during long-term follow-up of ICD implants.