Most sinonasal lesions that extend into the anterior cranial fossa are malignant tumors. However, benign inflammatory polypoid disease can also rarely be manifested in this manner. Fourteen surgically proved cases of such benign and malignant disease were studied with magnetic resonance (MR) imaging. The appearances of the benign and the malignant lesions were quite different and allowed the seven chronic polypoid lesions to be distinguished from the tumors. The benign lesions had nonhomogenous appearances on MR images due primarily to varying degrees of free-water resorption and protein concentration. The tumors had primarily homogeneous low-to-intermediate signal intensities due to their highly cellular composition.