Two personally observed cases of Castleman's benign lymphoma of the hyaline vascular type located in the mediastinum are described. The stages in the development of our knowledge of this tumour are traced and the various aetiopathogenic theories discussed. The difficulty of preoperative diagnosis is emphasised while the potential predictive value of calcifications combined with hypervascularization in an asymptomatic mass usually in the mediastinum and revealed by CT is pointed out.