Endless loop tachycardia (ELT) is a possible complication in dual chamber pacing; it is usually prevented by programming the atrial refractory period (PVARP) longer than the retrograde ventriculoatrial (VA) conduction interval; this in some patients limits the upper rate. In 15 patients with a DDD (nine patients) or a single-pass lead VDD pacemaker (six patients) and retrograde atrial activation, telemetric recording documented a significant difference in amplitude of antegrade, and retrograde atrial potentials (VDD 1.21 +/- 0.32 mV vs 0.56 +/- 0.23 mV, P = 0.008; DDD 2.7 +/- 1 vs 1.8 +/- 1 mV, P = 0.038; Student's t-test for paired data). In 3/15 patients ELT stopped after programming of atrial sensitivity to a value greater than the retrograde P wave amplitude; in 11/15 patients this occurred at a sensing value lower than or equal to retrograde P wave amplitude with a high pass band filter operating. One patient required PVARP lengthening. Holter monitoring showed no more ELTs. In most patients with a DDD or single-pass lead VDD pacemaker with widely programmable sensing amplitude and Hi/Low bandpass filters, individual programming of atrial channel sensitivity prevents ELT without affecting the PVARP and, consequently, upper rate limit.