The expanded indications for the use of implantable cardioverter defibrillators (ICDs) for primary prevention of sudden cardiac death have increased concerns about inappropriate delivery of therapies. Dual-chamber systems have improved the capacity of ICDs to differentiate ventricular arrhythmias from supraventricular arrhythmias. Still, T-wave oversensing is a major source of inappropriate therapy. It is likely the true incidence of T-wave oversensing is greater than reported as documented events reflect only those stored in a device memory. Reviewing cases of T-wave oversensing that failed noninvasive correction; we found successful resolution resulted from generator replacement. We conclude that T-wave oversensing is due to inadequate signal processing by some ICD generators.