Nasal and paranasal papillomas are rare benign tumors and are known with frequent local recurrence and occasional malignant change.The pathogenesis of these papillomas is not known, but recently a viral etiology has been proposed.Only a few ultrastructural observations of these papillomas have been reported by Gaito, R.A. et al. (1965), Jahnke V. (1971 etc.We studied papilloma in two patients ultrastructurally.Both patients were complaining of bilateral nasal obstruction, and a 68-year-old male had undergone removal of nasal papilloma twenty years ago and a 64-year-old male had nasal polypotomy three times in the past. Both were operated by Denker's method, and the specimens were taken at the time of biopsy and operation. Light microscopic findings were compatible of inverted papilloma. Specimens were immediately fixed in ice-cold buffered glutaraldehyde (Karnovsky) solution and post-fixed in 2 % osmium tetroxide solution buffered with cacodylate, then dehydrated in alcohol and embedded in Epon 812.Sections were cut and stained with toluidine blue for light microscopy.Ultrathin sections for electron microscopy were cut on a Porter-Blum Ultrotome. These were stained with uranyl acetate and lead acetate.Hitachi HU 12 type microscope (100 kV), and HU 11 B (75 kV) were used for examination.Electron microscopic findings were as follows :Case 1 : Superficial layer of flat cells and in some regions ciliated columnar cells were observed. Junctions between adjacent epithelial cells were consisted of cytoplasmic processes with desmosomes.Intermediated cells were polygonal in shape and had large nuclei and two or three nucleoli.The deeper layer of these cells were elongated and showed many mitosis and rested on discontinuous or obscure basement membrane. Peg like cytoplasmic process protruded into stroma. The intercellular space was gradually dilated in deeper layer.The cytoplasms of these tumor cells contained mitochondria, rough surfaced endoplasmic reticulum and abundant tonofilaments and a dense body in some regions.We could not demonstrate intranuclear virus like particles.Case 2 : Superficial layer was covered with flat cells, and intermediated cells had no atypia, mitosis, virus like particles, and the basement membrane was intact.It was thought that peg like process of basal cells indicated easy recurrence of this tumor, and slight atypia, mitosis and discontinuous basement membrane suggested precancerous condition.So we emphasized the utility of electron microscopical observation for differentiating nasal and paranasal papillomas either precancerous or not.