Objectives: To review all cases of atrioventricular septal defects in Northern Ireland from January 1990 to February 1999, examining clinical and morphological features, management, and outcome. Methods: A retrospective case note analysis of 106 subjects with comparisons between subgroups. Results: An atrioventricular septal defect was part of a more complex abnormality in 50 of the patients (47%). Down's syndrome was present in 57 (54%). Cardiac surgery was performed in 81%. The defects were unrestrictive in 69 patients (65%), 45 of whom had Down's syndrome. Complex associated abnormalities existed in 36 patients, and 10 of these died without cardiac surgery. Operative mortality was 9.5% for those with co-existing Down's syndrome group, and 14.3% for the chromosomally normal patients. The ventricular components of the septal defect were restrictive in 23 patients (22%), with 9 having Down's syndrome. Spontaneous closure occurred in more than half of these patients. Mortality was zero. The septal defect was exclusively at atrial level in 14 patients (“primum” defects – 13%), and 3 of these had Down's syndrome. Operative mortality was again zero. Median duration of postoperative follow-up was 3 and a half years. Overall, moderate to severe left atrioventricular valvar regurgitation was observed postoperatively in 23% at follow-up. Conclusions: Mortality was highest in the atrioventricular septal defects with an unrestrictive ventricular component. Uncomplicated cases had good outcomes. Patients without Down's syndrome tended to have more associated cardiac abnormalities, and to have more postoperative arrhythmias. Approximately half of the defects with restrictive ventricular components closed spontaneously. Moderate postoperative left atrioventricular valvar regurgitation was commonest in patients with the defect exclusively at atrial level.