Intranuclear inclusion bodies are sometimes observed in pulmonary adenocarcinoma by light microscopy. Electron microscopic characteristics of lung cancer cells with intranuclear inclusion bodies were studied. In addition, polymerase chain reaction (PCR) was performed using primers coding for human papillomavirus (HPV) types 16, 18, and 33. Eosinophilic intranuclear inclusion bodies were observed in 22 out of 285 cases by light microscopy. Immunohistochemically, cancer cell nuclei stained with PE-10. Three types of intranuclear inclusion bodies were classified electron microscopically. Type A showed aggregation of electron dense particles (30-40 nm) with an electron-dense core and was most frequently observed. Type B consisted of a mass of branching and whirling tubular structures. Type B intranuclear inclusions had a relationship with inner nuclear membrane. In type C, several spherical inclusions were observed in one nucleus. HPV DNA was detected using PCR and type-specific probes in a case with type A inclusion bodies. This study suggests that intranuclear inclusion bodies in pulmonary adenocarcinoma are formed by several different mechanisms.