OBJECTIVES: To identify determinants of adverse outcomes in patients with atrial isomerism. METHODS: Determinants of survival were analyzed for the group as a whole as well as separately for left and right atrial isomerism. RESULTS: There were 78 cases with right and 104 with left atrial isomerism. Nineteen patients were not offered surgery; 49 (47%) of those with left atrial isomerism and 60 (77%) with right atrial isomerism were directed to single ventricle palliation. A total of 67 patients died. Survival to 25 years was 62% (95% confidence interval [CI], 53%-69%). Independent predictors of mortality were obstructed total anomalous pulmonary venous drainage (P < .001; hazard ratio [HR], 5.2; 95% CI, 2.7-9.7) and asplenia (P = .008; HR, 2; 95% CI, 1.2-3.3). There was no evidence that patients born in the recent era had improved survival (P = .47; HR, 1.1; 95% CI, 0.8-1.5). Survival was better for patients with left than right atrial isomerism: 18 years survival 74% (95% CI, 63%-82%) versus 50% (95% CI, 38%-60%). Independent predictors of mortality for patients with left atrial isomerism were dextrocardia (P = .009; HR, 3.0; 95% CI, 1.3-6.7) and pulmonary stenosis (P = .042; HR, 0.3; 95% CI, 0.1-0.9) and patients with right atrial isomerism, biventricular repair (P < .001; HR, 6.0; 95% CI, 2.8-13), and obstructed total anomalous pulmonary venous drainage (P < .001; HR, 4.2; 95% CI, 2.0-8.6). CONCLUSIONS: A significant proportion of patients with isomerism still die before reaching adulthood. Only a fraction of those with obstructed pulmonary veins survive. Having biventricular repair does not confer a survival advantage to those born with right atrial isomerism.