The long-axis and short-axis two-dimensional (2-D) echocardiographie views from the parasternal window and the four- and two-chamber views from the apical window, respectively, are considered to be orthogonal to each other. The validity of this was tested using custom-designed 3.5-MHz transducers with an electric motor that rotated the transducer by 2.9° increments through 180°about a central axis. Images were acquired beginning at the best appearing longaxis parasternal or apical four-chamber views and rotating, blinded to the degree of rotation, to the best appearing short-axis or apical two-chamber views. Imaging in 40 patients (age 15-89, mean 46) undergoing clinical echocardiography yielded 35 parasternal and 27 apical rotations adequate for analysis. The difference between ‘orthogonal’ views in degrees of rotation were:(mean, minimum and maximum, respectively) from the parasternal window,100 ± 19°, 64° and 156°, and from the apical window, 95 ± 21°, 30° and 136°. Deviations from orthogonality exceeded 20° in 12/35 (34%) parasternal rotations and in 7/27 (26%) from the apical window; deviation exceeded 30° in 4/35 (11%) and 5/27 (19%), respectively. Thus, on average, the assumption of orthogonality between parasternal long- and short-axis views and between the four- and two-chamber views is correct, but there is wide deviation from this expectation in individual patients. These deviations may affect the accuracy of 2-D echocardiographie measurements and calculation of derived variables by methods that assume orthogonality of projections.