A 38-year-old man, who was diagnosed nasopharyngeal carcinoma (stage T4N2Mx) had rapidly growing lower neck mass (at level IV area) with local inflammation sign during concurrent chemoradiotherapy. After we performed Color Doppler sonography and fine needle aspiration biopsy of the neck mass, we differentiated neck soft tissue infection from tumor extension. Size of the mass decreased after antibiotics therapy. It was difficult to differentiate neck infection from tumor progression during treatment in naspharyngeal carcinoma.