In order to further understand the natural history and timing of repair for subpulmonic (type I) ventricular septal defect (VSD), 70 patients who underwent radical surgery were reviewed. The patients with this type of VSD accounted for 30 per cent of a total 236 children with types I, II and III VSD who underwent repair between 1978 and 1989. The frequency of aortic regurgitation was 30 per cent, being higher in the group aged older than 10 years than in the younger group (p less than 0.05). The median ages of patients with normal, prolapsed and regurgitated valves were 5, 6.5, and 10 years, respectively, and each value exhibited significant differences (p less than 0.05). The left to right shunt ratio and systolic pulmonary artery pressure of the patients with normal valves were significantly higher than those of the patients with abnormal valves. The outcome of the patients with regurgitation varied according to Sellers grading with cases of grade two valvular deformities recovering well after the repair, while cases of grade three had deformities of grade one or two severity remaining. These results led us to establish the following strategy for patients with I-VSD: 1) a child should be regularly checked by USG; 2) a child noted as having a prolapsing valve is recommended to undergo repair by the age of 10 to 12 years; and 3) a child found to have regurgitation should undergo repair as soon as possible, unless the valve regurgitation does not become more severe than Sellers grade two.