Nasal polyps are usually no diagnostic problem. From the histological aspect they can be divided into four categories: allergic, chronic inflammatory, fibrous polyp, polyp with hyperplasia of the seromucinous glands and polyp with atypical stroma. In the group of polyps with atypical stroma situations may develop where the assessment of the diagnosis is more complicated. This applies e.g. to cases where there is a not quite typical unilateral affection of the maxillary sinus, where on the CT picture a skeletal defect is apparent. This arouses suspicion of a malignant disease. Even histological diagnosis is not always simple, atypical stroma cells may be present which arouse in case of a clinical finding iitumor cavi nasi et sinuum paranasaliuml. suspicion of malignity. The most reliable differential sign is the absence of mitotic activity. The authors present the case-history of a 22-year-old woman and a 23-year-old man. Both had unilateral polypous to tumourous changes in the nose, on the CT obturation of the maxillary sinus with defects in the bony border of the maxillary sinus, biopsy of the nasal tissue was suspicious of neoplastic disease. In case of the female patient, based on the biopsy taken during sinotomy, chemotherapy on account of sarcoma of the sinuses was started and only after revision of the histological preparations the diagnosis was changed to atypical polyp. The authors assume that an inflammatory pseudotumour of the maxillary sinus with propagation into the nose be classified as an atypical polyp.

        Key words: atypical nasal polyp, histological classification of nasal polyps, inflammatory pseudotumour of the maxillary sinus.