Since June 1985 until April 1989, 237 patients (130 males, 107 females, aged 22 to 95 years, mean 71) with symptomatic AV conduction disturbances and competent sinus node, were implanted with a single-pass lead VDD pacing system in 30 centers and followed-up for at least 6 months. The ventricular pacing lead incorporated two atrial ring 3-cm apart electrodes, positioned within the right atrial cavity without contact with the heart wall, in order to detect the atrial activity, which is differentially processed by the pacemaker. At implant, mean atrial electrogram amplitude, derived from a custom pacemaker system analyzer (PSA) with the same input filter of the pacemaker was 1.7 +/- 0.8 mV (n = 93). In all cases, atrial sensitivity at implant was the default value +/- 0.15 mV. The atrial tracking capability of the pacing systems was assessed within the month and every 6 months after implantation by means of clinical evaluation, resting ECG, 24-hour Holter monitoring and the following tests: exercise stress testing, mental stress, isometric exercise, and nifedipine test. These tests evoke an increase of atrial rate in consequence of metabolic needs or as a reflex response. The criterion used to evaluate the correct operation of the system was the percentage of atrial synchronization. This was defined as the ratio between atrial triggered ventricular paced complexes and all ventricular paced complexes. All monitorings showed a ratio higher than or equal to 98% in a percentage of patients not lower than 95%. Mean follow-up was 385 days (range 183-1,370 days).(ABSTRACT TRUNCATED AT 250 WORDS)