The incidence and severity of atrial septal defects following balloon mitral valvuloplasty have been assessed using transthoracic and transoesophageal echocardiography in 20 patients 3-36 months following the procedure. In eight patients (group A) the atrial septum was dilated with an 8 mm Olbert balloon and either a double or bifoil balloon used to dilate the mitral valve. In 12 patients (group B) the Inoue balloon, with a slimmer deflated profile, was used following dilatation of the interatrial septum with a 14 French vessel dilator. In group A, using transthoracic echocardiography, one atrial septal defect was imaged and transatrial flow detected by colour flow Doppler in five patients. In seven of the eight patients transoesophageal echocardiography clearly imaged an atrial septal defect and left-to-right shunting was demonstrated by colour flow Doppler. Valsalva contrast studies revealed residual transatrial flow in all eight patients. The mean width of the colour flow jet was 5.8 mm. In group B patients, using transthoracic echocardiography, only one patient had evidence of residual transatrial flow (demonstrated by Valsalva contrast). Using transoesophageal echocardiography Valsalva contrast studies, transatrial flow was seen in 11 of the 12 patients. However, no defects were imaged and colour flow Doppler indicated left-to-right shunting in only two patients. The mean width of the colour flow jet was 1.5 mm. Transoesophageal echocardiography with colour flow Doppler and Valsalva contrast studies therefore provides a sensitive method for the detection of residual atrial septal defects following balloon mitral valvuloplasty. Transatrial flow persists in the majority of these patients.(ABSTRACT TRUNCATED AT 250 WORDS)