The authors report their experience with a case of bilateral pleural effusions as a complication of intrathoracic goitre. They then go on to examine all the possible causes of this phenomenon. After discussing the anatomy of the superior mediastinum, they conclude that hydrothorax related to intrathoracic goitre could be the result of pressure of the mass on venous structures, especially on the superior vena cava, or on the intramediastinal lymphatic vessles, or, as in the case reported, on both.