The objectives of the study were to analyze the clinical features of inverted papillomas (IP) associated with malignancy and to evaluate the correlation of tumor stage, survival and histolologic features. We conducted a retrospective review of 18 IP associated with malignancy patients. In addition, we compared histopathologic characteristics (tumor differentiation and malignant cell proportion) with clinical outcomes. Eleven of the tumors were present on the nasal cavity and 7 on the maxillary sinus. The rates of synchronous and metachronous malignancy were 10.1 and 1.1%, respectively. The disease-free survival rate was 83.3%. The tumors were staged as T1 (5/18), T2 (2/18), T3 (8/18), and T4 (3/18). According to the percentage of the malignant cell in the entire tumor tissue, 4 patients (22%) were in grade I, 4 patients (22%) were in grade II, 3 patients (17%) were in grade III, and 7 patients (39%) were in grade IV. There was no relationship between recurrence- and/or disease-free survival and histologic findings including tumor differentiation and malignant proportion. IP-associated malignancy tends to occur synchronously and have more favorable prognosis compared to other sinonasal malignancy. Furthermore, the proportion of malignant cell to IP and tumor stage seems not to affect the clinical outcome of IP-associated malignancy.