We report the case history of a patient with a sphenoid sinus mucocoele detected by computed tomography and medical resonance imaging. The patient had a history of nasopharyngeal carcinoma, which was treated by radiotherapy more than 10 years previously. He presented with bilateral twelfth and sixth cranial nerve palsies. Local tumour recurrence was suspected. Further investigations showed that the cranial nerve palsies were caused by radiation damage and the sphenoid sinus mucocoele was an incidental finding. Sphenoid sinus mucocoele is a possible rare late complication of radiotherapy in patients with nasopharyngeal carcinoma.