The hospital mortality rate was 9.1 per cent (three deaths) in 33 patients less than 2 years old undergoing primary repair of single large ventricular septal defects (VSD's) since January 1972, and was not related to age; it was zero in 50 patients older than 2 years (Group A). The mortality rate with multiple VSD's was 36.4 per cent, not related to age. Age at operation, preoperative pulmonary vascular resistance, and pulmonary artery pressure were directly related to mean pulmonary artery pressure 5 or more years later in 74 patients (Group B) operated upon between 1955 and 1968. Over-all probability of "surgical cure" for single large VSD (surviving the early and late postoperative period with normal or near-normal pulmonary artery pressure 5 years later) is 92 per cent when pulmonary vascular resistance is 4 units - sq. M. and operation performed at age 27 months, 87 per cent when resistance is 8 and operation done at age 6 months, and 80 per cent when resistance is 12 and operation done at less than 6 months of age. These and other probability curves are examined and compared with natural history to determine optimal age for elective repair for large VSD's.