125 patients with an incomplete type of atrioventricular canal defect (AVC) were followed up over a very long period. A study of the changes in parameters which are not open to great variability has allowed a more precise approach to the spontaneous and postoperative course of this congenital cardiac defect. The fact that the condition is usually well during the first three decades is not a justification for witholding operation; complications occur after the thirtieth year, and the operative risk becomes greater. Although only 41 patients in this series underwent surgery, we have been able to assess the surgical results and, as has been reported in the literature, these appear to be good. It is therefore necessary to extend the indications for surgery to include all the incomplete forms of AVC defect, and especially to operate during childhood or adolescence in order to minimise the risks of surgery.