RESUMEN
INTRODUCTION: The malignant mesenchymoma is a malignant tumour composed of two or more types of non-differentiated tissue, associated with fibrosarcomatous elements. Its mediastinal localisation is exceptional. OBSERVATION: In a 65 year-old woman, recurrent pericardial effusion revealed a malignant mesenchymoma measuring 11 x 9 x 4 cm, located in the mediastinum and extending towards the pericardium. The histological examination of the surgical piece showed the predominance of an osteo-sarcomatous component. Complete resection was performed with partial pericardectomy, followed by adjuvant radiotherapy. CONCLUSION: Malignant mediastinal mesenchymoma is an exceptional tumour. Its diagnosis is based on anatomopathological study of a mass of anarchic composition, and its poor prognosis is related to its localisation and its capacity to relapse locally.