By electron microscopy, viral particles were searched for in specimens taken from 3 cases of verruca vulgaris, 2 cases of verruca plana, 2 cases of epidermodysplasia verruciformis, 8 cases of oral papilloma, 2 cases of nasal papilloma, 4 cases of laryngeal papilloma, 2 cases of condyloma acuminatum, 3 cases of bladder papilloma and 1 case of familial intestinal polyposis. Viral particles were detected in all 3 cases of verruca vulgaris, 2 cases of verruca plana, 2 cases of epidermodysplasia verruciformis and in one of 2 cases of condyloma acuminatum. All the viral particles detected in these papillomas were similar morphologically and antigenically and appeared to belong to the papilloma virus group. Vacuolation of cells in the upper epidermal layers was observed histologically in both virus-positive and virus-negative papillomas, and the histologically recognizable characteristics of virus-positive papillomas appeared to be the swelling of the nucleus, peripheral accumulation of the chromatin and clearly distinguished nucleoli in vacuolated cells. These nuclear changes were also observed electron microscopically. Malignant histological features were observed in 1 case of epidermodysplasia verruciformis, 1 case of oral papilloma, 1 case of laryngeal papilloma, and 1 case of familial intestinal polyposis. The significances of these findings are discussed.