The natural history of aortic cuspal prolapse and aortic regurgitation (AR), studied most commonly in subpulmonic ventricular septal defect (VSD), has not been well defined in isolated, unrepaired VSD diagnosed during infancy. This study aimed to define the incidence and progression of aortic cuspal prolapse and AR in patients with subaortic VSDs diagnosed at <1 year of age who had no aortic cuspal prolapse or AR at presentation and did not require surgery within the first year of life. Patients had yearly follow-up, and data regarding clinical course, physical examination, and echocardiography were obtained. Comparisons were made between patients who developed aortic cuspal prolapse and AR and those who did not. One hundred patients, with a mean age at VSD diagnosis of 0.1 +/- 0.5 years, followed for a mean of 7.1 +/- 10.1 years, were studied. Aortic cuspal prolapse developed in 14 patients (14%) at a mean age of 7.1 +/- 6 years (range 0.4 to 18.4). AR murmurs were heard in 6 patients (6%) at a mean age of 5.1 +/- 3.1 years, all of whom had aortic cuspal prolapse and underwent VSD closure and aortic valvuloplasty. In conclusion, aortic cuspal prolapse and clinical AR are not uncommon in patients with subaortic VSDs. Long-term follow-up of patients with subaortic VSDs should include the serial evaluation of aortic valve anatomy and function.