With the increasing recognition of the pacemaker syndrome and with the availability of newer methods of pacing therapy, the status of the atrioventricular (AV) conduction in patients with Sick Sinus Syndrome (SSS) becomes crucial in the choice of mode of pacing. At the Philippine Heart Center, from April 1983 to November 1986, the sinus and AV node function studies of 46 patients who, by electrophysiological studies had sinus node dysfunction (SND)-SN recovery time (SNRT) greater than 1400 msec, were reviewed. These were arbitrarily classified according to duration of SNRT into: Group A--borderline SND, SNRT from 1,401 to 1,499 msec (n = 4); Group B--mild SND, SNRT from 1,500 to 2,499 msec (n = 25); Group C--moderate SND, SNRT from 2,500 to 3499 msec (n = 6); Group D--severe SND, SNRT of 3,500 m and above (n = 11). Out of the 46, 14 (30%) had concomittant AVN dysfunction (AVND)--antegrade block rate less than 130 beats per minute (BPM). The percentage occurrence of AVND was noted as follows: Group A--25% (1/4); Group B--28% (7/25); Group C--50% (3/6); Group D--27% (3/11). Out of the 14 patients with concomittant AVND, 5(35%) had antegrade block rate less than 100 BPM, 3 in Group B and 2 in Group D. The study shows that AVND occurs in only 30% of SSS patients. Its occurrence and severity has no bearing on the degree of SND. In these, antegrade block was at a rate higher than 100 BPM in the majority of patients.(ABSTRACT TRUNCATED AT 250 WORDS)