A previously unrecognized form of failure of normally functioning noncompetitive R-inhibited or R-triggered pacemakers to sense is described in 30 patients. The failure was produced by the delayed arrival of ventricular depolarization due to intraventricular conduction disturbances at the site of pacemaker sensing electrodes. These patinets with late activation of the right ventricle exhibited right bundle branch block (RBBB) during conducted sinus beats or had ectopic beats arising from the left ventricle. In both these situations, several late occurring QRS complexes were not sensed by the pacemaker electrodes in the right ventricular apex (RVA) and thereby the R-inhibited pacemaker spike discharged into the initial portion of the QRS complex. Activation of the RVA was delayed up to 65 msec in sinus beats with RBBB and even longer in premature beats arising from the left ventricle. The depolarization front of such beats reached the myocardium around the right ventricular pacemaker electrodes late in the QRS and thereby were not sensed up to 65 msec after the QRS onset as seen on the surface electrocardiogram (ECG). It was found in this study that if the inscription of such QRS complexes began up to 65 msec before the next due pacemaker impulse, these complexes were not sensed, allowing discharge of normally functioning R-inhibited pacemakers within the QRS which mimicked pacemaker malfunction. Failure to sense due to this phenomenon was observed in patients with noncompetitive pacemakers, both R-inhibited and R-triggered; temporary and permanent; with unipolar as well as bipolar electrodes. Similarly, patients with left ventricular epicardial electrodes and left bundle branch block in sinus beats also exhibited this phenomenon. Complete evaluation proved that pacemaker function was normal in the entire patient group. Recognition that failure to sense was the result of intraventricular conduction disorder prevented the untimely replacement of the pulse generator. This phenomenon of nonsensing by noncompetitive pacemakers should be considered in the presence of wide QRS complexes before the pacemaker is concluded to be malfunctioning.