To validate the usefulness of the subcostal acoustic window (inferior vena cava-right atrial projection) for the echocardiographic diagnosis of pericardial effusion we studied 40 patients with pericardial effusion detected through the parasternal approach. A group of 100 patients without evidence of effusion constitutes the control group. In all patients in the control group, a contact between the right atrial wall and the diaphragm was noted during the cardiac cycle (specificity: 100%). This had also been detected in 15 of 18 patients with mild effusion and in only 1 of 11 patients with moderate effusion, and in none of 11 patients with severe effusion (overall sensitivity: 60%). Therefore, moderate and severe pericardial effusions can be detected in most cases (95.4%) below the right atrial wall through the subcostal inferior vena cava-right atrial projection. The echocardiographic criteria for its diagnosis is the lack of contact through the cardiac cycle between right atrial wall and the diaphragm.