Interatrial block (IAB; P-wave duration>or=110 ms) is highly prevalent and is strongly associated with atrial tachyarrhythmias and left atrial dysfunction. Very few studies have examined IAB in young healthy subjects. The aim of the present study was to demonstrate the prevalence of IAB and its possible relation with clinical variables in 1,353 young healthy men. It was found that 9.1% of healthy men aged<35 years and 5.4% of those aged<20 years had P-wave durations>or=110 ms. The frequent presence of IAB in leads II, V3, and V5 was also observed. Age and heart rate were independent significant determinants of IAB. In conclusion, IAB is a frequent phenomenon, even at young ages. Thus, the early recognition of IAB might be important, possibly contributing to the prevention of future cardiovascular complications.