Contrast M-mode and two-dimensional ultrasonography of the inferior vena cava were performed in 65 patients with various acquired and congenital cardiac disorders. After saline was injected into a peripheral arm vein, the inferior vena cava was visualized by both methods in 60 patients (92%). The M-mode approach was better for correlating the appearance of contrast within the inferior vena cava with the ECG. This precise correlation served as the basis for differentiating patients with tricuspid insufficiency (visualization of contrast after the QRS) from those with impaired right ventricular filling (visualization of contrast before the QRS) or arrhythmia. Results with the M-mode approach suggest that conditions other than tricuspid insufficiency may cause the appearance of contrast within the inferior vena cava. M-mode echocardiography should be used to further investigate patients with impaired right ventricular filling.