The patient reported here had no particular past history. He presented with effort dyspnea, jugular turgescence and positional vertigo suggestive of a superior vena cava obstruction syndrome. Bronchial fibroscopy was normal, but chest X-ray, CT scan and phlebography of the superior vena cava revealed an anterior mediastinal tumour compressing the superior vena cava. The tumour was excised with replacement of the vena cava by a PTFE n. 16 graft. Pathological examination showed this to be an inflammatory tumour by fibrosis with plasmocytes. This plasmocyte fibrosis is histologically benign and becomes malignant only when complications occur as a result of its spread.