Thymic cysts are rare, generally asymptomatic lesions, which in most cases are discovered incidentally on the chest x-ray and are localized in the anterior compartment of the mediastinum or in the neck. We report the case of a 41 year old man, who developed a compression of the left lung and a protuberance of the left chest wall as a result of multilocular thymic cyst with an exceptional volume of 2 litres.