Ventricular septal defects - large, surgically closed or small, untreated - have demonstrated lower peak exercise capacity compared with healthy controls. The mechanisms behind these findings are not yet fully understood. Therefore, we evaluated biventricular morphology in adults with a ventricular septal defect using MRI. Adults with either childhood surgically closed or small, untreated ventricular septal defects and healthy controls underwent cine MRI for the evaluation of biventricular volumes and quantitative flow scans for measurement of stroke index. Scans were analysed post hoc in a blinded manner. In total, 20 operated patients (22±2 years) and 20 healthy controls (23±2 years) were included, along with 32 patients with small, unrepaired ventricular septal defects (26±6 years) and 28 controls (27±5 years). Operated patients demonstrated larger right ventricular end-diastolic volume index (103±20 ml/m2) compared with their controls (88±16 ml/m2), p=0.01. Heart rate and right ventricular stroke index did not differ between operated patients and controls. Patients with unrepaired ventricular septal defects revealed larger right ventricular end-diastolic volume index (105±17 ml/m2) compared with their controls (88±13 ml/m2), p<0.01. Furthermore, right ventricular stroke index was higher in unrepaired ventricular septal defects (53±12 ml/minute/m2) compared with controls (46±8 ml/minute/m2), p=0.02, with similar heart rates. Both patient groups' right ventricles were visually characterised by abundant coarse trabeculation. Positive correlations were demonstrated between right ventricular end-diastolic volume indices and peak exercise capacity in patients. Left ventricle measurements displayed no differences between groups. In conclusion, altered right ventricular morphology was demonstrated in adults 20 years after surgical ventricular septal defect repair and in adults with small, untreated ventricular septal defects.