In 30 symptomatic patients with complete atrioventricular block, sinus node recovery time was measured in order to establish the incidence of sinus node dysfunction. Prolonged corrected sinus node recovery time (more than 560 msec) was found in seven cases (23%). Sinus node recovery time expressed as percentage change of the basic cycle length exceeded the upper limit of normal (more than 170%) in nine cases (30%). The mean value of these parameters was 563 msec and 179%, respectively. The study shows that sinus node dysfunction frequently coexists with complete atrioventricular block.