Primary leiomyosarcoma of the thorax is a rare malignant mesenchymatous tumour. Most of those tumours occurring in the mediastinum arise from the oesophagus and great vessels, whereas an appearance in the soft tissue of the mediastinum is extremely rare. The rare incidence of this tumour and its slow growth reflect the difficulties in differential diagnosis according to their histopathology and location. The most common treatment is surgical resection. The authors describe a case of a leiomyosarcoma arising from soft tissue of the mediastinum and present a review of the literature.