Five cases of sinonasal undifferentiated carcinoma (SNUC) were treated between 1987 and 1991 in the ENT department of National Taiwan University Hospital. The patients, ranging in age from 35-59 years, had relatively few symptoms at presentation. Three patients died from 4 to 8 months after diagnosis (mean: 6 months) and two are alive without disease with a follow-up period of at least 6 months. Microscopically, a high mitotic rate, tumor necrosis and prominent vascular permeation were characteristic. Sinonasal undifferentiated carcinoma is a rare, highly aggressive neoplasm that may be present in patients who had symptoms significantly less than the amount of disease present on CT scan.