A 30-year-old male chemical plant worker had an unusual sinonasal lesion characterized by well-formed villi lined by resorptive, Paneth, argentaffin and goblet cells. The villi were underlain by smooth muscle resembling muscularis mucosa. In other areas, the pattern was more disorganized. Using criteria applied to analogous intestinal lesions, these foci were adenomatous, but cytologically benign. Nonetheless, both the villi and the adenomatous component invaded bone, and recurred multiple times. The cytologic blandness and well-organized architecture of this lesion are deceptive; they belie a lesion that is highly aggressive locally. Our patient died of disease 8 years after initial diagnosis.