In order to assess whether the paradoxical motion of the interventricular septum seen in patients with atrial septal defect (ASD) is due to a true abnormality in septal contraction, eight patients with ASD (age, 1.6-17 years) and eight age-matched control patients were studied using qualitative and quantitative two-dimensional (2D) and M-mode echocardiography. 2-D-echocardiographic images recorded from the parasternal short-axis projection at the level of the papillary muscles and 2D-directed M-mode tracings at this level were obtained. Comprehensive wall motion analysis of the left ventricular (LV) endocardial and epicardial borders was performed using both fixed reference and center of mass (floating reference) models. Our results indicate that interventricular septal wall motion and function are normal in patients with ASD. The apparent "paradoxical" motion is due to excessive anterior motion of the entire left ventricle, and is present only when a fixed reference system is used to assess myocardial motion, but is not present when a center of mass (floating reference system) is employed. Left ventricular function assessed by % area and perimeter change, mean radial shortening fraction, and mean radial wall thickening (2D) as well as LV shortening fraction and septal and posterior wall thickening (M-mode) was not significantly different between the two groups. Standard M-mode tracings can therefore be used to assess LV function despite this apparent abnormal septal motion.