Postoperative cheek cyst(POCC) is a late complication of Caldwell-Luc's operation. It has various symptoms and patients complain the cheek symptoms, eye symptoms or dental symptoms. The diagnosis of POCC depends on patient's operation history, simple PNS X-rays, CT scan or MRI scan. But there are problems in diagnosing the POCC. We performed the sonogram for diagnosing the POCC in 13 patients. Sonographic findings of POCC are an anechoic mass filling the maxillary sinus and buldging cyst through anterior surgical bony defect. It is our opinion that ultrasound may be the next modality followed by positive simple PNS X-rays.