The resting 12 leads electrocardiogram was recorded in 3,822 men and 4,969 women (aged 30 years or over), who participated in the First National Health Examination Survey of Thailand and the data was used to determine the prevalence of cardiac arrhythmias. In this apparently normal population 362 subjects (the standard age adjusted rate was 39.2 per thousand) had cardiac arrhythmia. The prevalence rate of atrial fibrillation was 3.6 per thousand (males 3.6, females 3.6) and ventricular premature beat was 12.1 per thousand (males 7.3, females 15.6). For atrial premature beat, complete and incomplete right bundle branch blocks, sinus arrhythmia, first degree atrioventricular block and delta wave, the prevalence rates were 4.0, 7.0, 4.6, 2.7, 2.4 and 1.3 per thousand respectively. Complete and incomplete left branch blocks, left anterior hemiblock, second degree antrioventricular block and short PR interval were rare in this survey.