The time interval between mitral and tricuspid valve closure was measured from M-mode echocardiograms in patients with Ebstein's anomaly. It was found that this time interval demonstrated a range of values within each patient. The use of different transducer positions on the chest wall was found to be the predominant factor. There was no correlation between the measured time interval, its variability, and the moment of measurement during breathing. However, a period of breathholding significantly reduced the variability. This study indicates that a wide range of time intervals might be measured in a single patient. We conclude therefore, that this parameter should be used with great care in the diagnosis of Ebstein's anomaly.