OBJECTIVE--To assess the morphology of fossa ovalis atrial septal defects (FOASD; secundum atrial septal defect) to determine in what proportion percutaneous closure with the clam-shell device might be feasible. DESIGN--Review of the intraoperative description of the morphology and size of FOASD. PATIENTS--106 consecutive patients with FOASD. SETTING--A regional children's cardiac referral centre. MAIN OUTCOME MEASURES--Determination of morphology and size of FOASD. RESULTS--Morphological variations in the FOASD were found: (1) central FOASD, 70 (66%); (2) superior FOASD, 4 (3.7%); (3) inferior FOASD, 8 (7.6%); (4) posterior FOASD, 2 (1.9%); (5) subtotal absence of atrial septum, 3 (2.8%); (6) completely fenestrated FOASD, 10 (9.4%); (7) partially fenestrated FOASD, 9 (8.5%). Depending on its site, the edge of the FOASD may be close to important atrial structures such as the orifices of the superior and inferior caval veins, coronary sinus, and right pulmonary veins. The shape of the FOASD was usually oval with the major diameter ranging from 10 to 50 mm with a mean (SD) of 27.8 (0.93) mm. The minor diameter ranged from 4 to 30 mm with a mean (SD) of 15.3 (5.9) mm. The ratio of the major to minor diameter ranged from 1.0 to 5.0 with a mean of 2.01. Based on intraoperative measurement and description, it is postulated that it should be possible to close about 50% of these defects with the clam-shell device.