Sinus node dysfunction may be clinically asymptomatic or may lead to serious arrhythmias and sudden death. Symptomatology relates to the resulting brady- or tachyarrhythmias. Clinical and electrocardiographic assessements fail to identify the cause in many patients. Ambulatory electrocardiography establishes the diagnosis in a greater percentage of patients and permits the direct correlation of symptoms with rhythm disturbances. Functional electrophysiologic testing which examines the sinus node response to constant atrial pacing and premature atrial stimulation may be performed for further evaluation of these patients. Sensitivity and specificity of sinus node recovery times (SNRT) and sino- atrial conduction time (SACT) derived by functional testing vary markedly in different reports. This variation may relate to differences in patient population, limitations of testing procedures, and the uncertainties as to the normal limits of SNRT and SACT. As a result, the full value of these tests in diagnosis and in furthering our understanding of sinus node disease remains to be established. At present, permanent pacing remains the definitive treatment in symptomatic patients.