We report a 29‐year‐old female who presented with thyrotoxicosis and a massive right pleural effusion. Cardiac catheterization in the presence of the effusion revealed equalization of abnormally elevated right and left heart filling pressures. Repeat study after thoracentesis demonstrated normal intracardiac pressures, which were unchanged by volume loading with normal saline. This case report suggests that both extrinsic cardiac chamber compression produced by the large unilateral effusion and ventricular interdependence altered diastolic function of the right and left ventricle in a manner resembling constrictive pericarditis or restrictive cardiomyopathy. Copyright © 1984 John Wiley & Sons, Ltd.