OBJECTIVE To investigate the cause of epistasis in patients irradiated for nasopharyngeal carcinoma. METHOD Seventy nasopharyngeal carcinoma cases with epistasis after radiotherapy were retrospectively analyzed. RESULT The causes of epistasis were local nasopharyngeal carcinoma recurrent 34.3% (24/70), pseudoaneurysm 14.3% (10/70) and nasopharyngeal crust 51.4% (36/70). Seven cases with local recurrent, 3 cases with pseudoaneurysm and 1 cases with nasopharyngeal crust and osteoradionecrosis died of intractable epistasis. CONCLUSION Epistasis in patients irradiated for nasopharyngeal carcinoma is dangerous complication, especially in cases with local recurrent and pseudoaneurysm. Nasopharyngeal crust must be treated actively.