We report a case of a neuroendocrine carcinoma arising in a wound of the postoperative maxillary sinus that was difficult to distinguish from a postoperative maxillary cyst. The patient was a 65-year-old Japanese woman who complained of left exophthalmos with cheek swelling and eye movement disorders. In past history, she had, 40 years previously undergone operation on the bilateral maxillary sinus by Caldwell-Luc's method. In a preoperative computed tomography, a mass occupied the left maxillary sinus showing irregular densities with destruction of the posterior bone walls and invasion into the left orbital. Both TI and T2 weighted magnetic resonance imaging showed low intensities and unevenness in the mass. We performed a biopsy of the maxillary tumor according to Caldwell-Luc's method. Histological examination diagnosed neuroendocrine carcinoma. Radiation therapy (total 66Gy) resulted in partial response for this tumor. However, sinonasal neuroendocrine carcinoma has been identified as highly aggressive, with a high probability of recurrence and metastasis.