31 cases of ASD with mitral incompetence have been reviewed. The aetiology of the mitral incompetence is most frequently congenital, contrary to the classical view. The usual mitral defect is a combination of gaping of posterior commissure with a narrow valvular opening. The usual angiocardiographic picture is one of prolapse of the mitral valve. One of the characteristic findings, which has already been reported in the literature, is of redundant valve tissue, with myxomatous degeneration (one case confirmed histologically). Surgery is indicated in every case. Conservative mitral surgery gives satisfactory results in the mid-term.