Three patients who had closure of an ostium primum defect developed severe fixed subaortic stenosis requiring surgical removal three to 11 years late. In two, the basic anatomy of the attachment of the superior portion of the anterior mitral cusp and submitral apparatus predisposed to the formation or deterioration of this complication. Both had subvalvar gradients and angiographic narrowing of the outflow tract in systole and diastole. In the third patient the strut of the Hancock prosthesis used to replace a double orifice mitral valve impinged on the outflow tract to stimulate the formation of a serious fixed obstruction. Subaortic gradients before operation must be specifically looked for, particularly in those with the characteristic radiological deformity of the submitral apparatus, and at operation the subaortic region must be inspected. Postoperatively the search for developing subaortic stenosis must include regular M-mode and 2-dimensional echocardiography, and cardiac catheterisation may be required. Although a rare complication, subaortic stenosis is a progressive and damaging lesion which requires early recognition and treatment despite the apparent well-being of the patient.