The purpose of this study was to investigate the incidence and clinical features of atrial septal defect (ASD) in school children in Japan who were diagnosed by heart disease screening. From 1989 to 1998, a questionnaire, electrocardiography (ECG) and phonocardiogram were obtained from school children when they entered their first year of elementary school (n=86,142) or junior high school (n=80,632). In this program, 33 asymptomatic ASD patients were newly diagnosed (0.020%). The ECG findings showed incomplete right bundle-branch block (79%), right axis deviation (55%), and right ventricular hypertrophy (9%). An ejection systolic murmur was audible in 30 patients (94%) and mid-diastolic murmur in 10 patients (30%). Thirty patients (90%) showed fixed split of second heart sound. Using echocardiography or catheter observation, 31 patients (94%) were judged to require closure of the ASD. Although the medical care is widely available in Japan, undetected ASD patients were not rare and importantly, most of them required closure of the defect even if they were asymptomatic. (Circ J 2003; 67: 112 - 115)