Objective: Our aim was to investigate the change in diameter of holes within the oval fossa, and the role of aneurismal formation in reducing the size of the hole, in patients diagnosed during infancy with isolated defects in the floor of the fossa. Patients: In a retrospective study, we included 100 patients diagnosed during the first year of life with an isolated defect in the floor of the oval fossa who had subsequently been observed for at least 5 years. There were 56 females and 44 males. They had been admitted to hospital because of a murmur in 65, heart failure in 9, and other reasons in 17. The remaining 9 patients were referred from other institutions with an established diagnosis of defects within the oval fossa. Patients were grouped according to the size of the deficiency in the floor of the fossa. Defects of diameter less than 5 mm were considered to be small, and 20 patients had such defects. Medium sized defects were judged to be between 5 and 8 mm, with 26 patients fulfilling this criterion, with the other 54 patients having large defects with diameters greater than 8 mm. Results: The overall spontaneous rate of closure was 27%. Of those with medium defects, half closed spontaneously, but only 7.5% of those with large defects showed such closure. Of the patients who were diagnosed with heart failure, 9 had defects measuring 7 mm, and of these, 6 required surgical closure, 1 patient had spontaneous closure, while the defect became smaller in the remaining 2. On the other hand, in 9 out of 10 patients who had aneurysms, the diameter of the defect was between 7 and 15 mm. Amongst these patients, the defect closed spontaneously in 3, and reduced in size in the others. Conclusion: When holes within the oval fossa measure 8 mm or below,, the majority of patients with experience either spontaneous closure or show decrease in size of the defect. In those with larger defects, the size usually increases, and surgery is needed for closure. If there is aneurismal formation, however, even when the defect measures more than 8 mm, the defect usually closes on its own or gets smaller.