Aim To perform late postoperative assessment of patients with ostium secundum defect. Methods We studied 45 patients 22±4 years after operation using clinical examination, transthoracic and transoesophageal echocardiography and electrocardiography. Results Patients operated on at ≤24 years had normal exercise tolerance while 11/17 (65%) patients operated on at >24 years reported dyspnoea upon exercise. Mitral regurgitation occurred more frequently in patients operated on at ≤24 years than at >24 years (29% vs 69%, P30mmHg was measured in seven patients (16%). Seventeen patients (38%) had significant electrocardiographic abnormalities. Conclusions Late after uncomplicated seclusion of ostium secundum defect patients operated at >24 years have more symptoms than those operated on at an earlier age. Residual lesions are common. Mitral regurgitation is more frequent in patients operated on at >24 years. Our findings support the clinical consensus of operating on these patients in their childhood and adolescence.