Primary parapharyngeal space tumours are rare, but they pose not only diagnostic but also therapeutic challenges in head and neck surgery. Imaging studies, in particular magnetic resonance (MR) imaging, play a central role in the diagnosis of parapharyngeal space (PPS) tumours. Besides schwannomas, primary lesions arising from the sympathetic chain within the PPS are extremely rare. We describe a 49-year-old man in whom the cervical sympathetic ganglion became enlarged after radiotherapy for nasopharyngeal carcinoma, and appeared as a parapharyngeal mass. This phenomenon has not been reported in the literature. We also discuss the features of the enlarged cervical sympathetic ganglion on MR imaging.