Sensing of far-field QRS complex through the atrial pacemaker lead may cause a number of pacemaker function disturbances, most of which are rarely seen with modern pulse generators. However, certain pulse generator algorithms will still be jeopardized by far-field QRS complex sensing. Intracardiac electrograms with markers were obtained by telemetry in 30 patients following implantation of a permanent bipolar atrial lead and a DDDR pulse generator. The occurrence and timing of far-field QRS complex sensing was studied at different atrial amplifier sensitivity settings. With paced ventricular complexes, QRS sensing was documented in all 30 cases at the maximum atrial sensitivity (0.1 mV). The median QRS complex sensing threshold was 0.3 mV, and the sensing window at high atrial sensitivities was 67-202 ms following the ventricular pacing impulse. In one case, QRS complex sensing was seen up to an atrial sensitivity of 1.5 mV. In 12 of 13 patients with 1:1 AV conduction, atrial sensing of spontaneously conducted ventricular complexes was seen (median sensing threshold 0.2 mV; the sensing window was -23 to 114 ms relative to the ventricular amplifier sensing event). Far-field QRS complex sensing was also found in all 12 patients in whom ventricular fusion complexes were obtained (median sensing threshold 0.2 mV; the window of sensing was 64-187 ms after the ventricular pacing impulse). Constant or intermittent QRS complex sensing via the atrial bipolar lead was thus universally demonstrable. It occurred in only a minority (20%) of patients at a sensitivity of 0.5 mV or less. Knowledge regarding the timing of the oversensing as related to the atrial sensitivity setting may aid in the design of algorithms of future pacemakers and cardioverter defibrillators.