Eight patients (7 females and one male) ranging in age from 6 to 30 years (mean 11.2 years), with secundum-type atrial septal defect (o.s. ASD) and cleft mitral valve (CVM) were surgically corrected at the Cardiac Surgery Department of Massa Hospital (Massa-Italy) from 1974 to 1981. All patients were studied with non invasive diagnostic techniques and with cardiac catheterization and angiography. All had mitral regurgitation of variable degree. On the ECG, there were P and QRS wave abnormalities but no superior quadrant QRS axis deviation suggestive of endocardial cushion defect. At operation the cleft(s) was found in variable positions: on the anterior leaflet in 4 cases and on the posterior leaflet in four. In two cases there was a double cleft. Correction was accomplished in 5 cases with direct repair of CVM, associated with various types of anuloplasty and in 2 cases anuloplasty only was done, without suturing the cleft. The septal defect was closed by direct suture in one case and with a Dacron patch in the remaining seven. There was one early death in the first 24 hour post-operatively. The seven surviving patients have been followed for a period of time ranging between 2 and 93 months (mean 29 months). No survivor has been re-evaluated by cardiac catheterization and cineangiography. No patient shows clinical or non-invasive evidence of residual mitral regurgitation behind grade I (slight). We conclude that: 1) the CVM associated with o.s. ASD cannot be securely diagnosed preoperatively with the common diagnostic techniques, even when producing severe mitral regurgitation, except perhaps by B-mode echocardiography; 2) the CVM (part of the spectrum of endocardial cushion defect malformations) in no cases was associated with the typical ECG or angiocardiographic abnormalities; 3) the CVM associated with o.s. ASD should always be repaired because, compared to the CVM that occurs with endocardial cushion defects, its site and regurgitation characteristics are variable, and simple ASD closure may not be sufficient.