A review of 108 cases of coarctation of the aorta diagnosed under 1 year of age, and admitted to The Hospital for Sick Children, Toronto, over a 12-year period, is presented. Preductal coarctation accounted for 90% of the fatalities under 1 year. Patency of the ductus arteriosus occurned in 98% of cases in the first 6 months of life. Forty-nine per cent of cases of coarctation of the aorta under 1 year of age have major cardiac anomalies other than patent ductus: ventricular septal defect, 33%; transposition of great vessels, 10%; and atrial septal defect, 6%. Problems in diagnosis occurred when blood pressure readings were equivocal. Operative mortality for coarctation of the aorta in the first year of life was 41%, while after 1 month of age it was only 29%. Mortality in patients not operated on during the first year of life was 50%, while for those less than 1 month of age with symptoms and signs of decompensation and not operated on, the mortality was 87%. In dealing with coarctation of the aorta in the first year of life the following are recommended: 1) When blood pressure readings in the arms and legs are equivocal, the readings should be repeated frequently until the diagnosis is clarified; 2) aortogram should be performed on patients difficult to diagnose; 3) babies presenting with symptoms in the first month of life should be operated on promptly unless they show a dramatic response in a 12-hour trial of treatment with digitalis; 4) babies after 1 month of age who respond to digitalis may be kept on this medication until adequate adjustment in circulatory hemodynamics has taken place and surgery can be performed at the optimum age.