Ganglioneuroma is a very rare tumour characterised by a slow evolution and arising from cells originating in the neural crest. This type of tumour occurs most commonly in the posterior mediastinum and is considered malignant in childhood and benign thereafter. It grows very slowly and can usually grow to a substantial size without giving rise to an evident clinical picture. We report the case of an 18-year-old asymptomatic patient, who had undergone a medical examination in a military hospital where a chest X-ray revealed a roundish mass in the right lung displacing the trachea to the left. Computed tomography showed that the mass was located in the posterior mediastinum. Surgical resection was performed. The tumour was encapsulated and adhered tightly to the pleura of the costovertebral angle. Histological examination revealed a ganglio-neuroma composed of Schwann cells and mature ganglion cells without any neuroblastomatous elements. This case of a mediastinal ganglioneuroma in a young male patient is a typical example of this rare pathological entity. The totally asymptomatic clinical picture could be explained by the very slow growth of this type of tumour which can displace the surrounding anatomical structures without infiltration.