The presence of human papillomavirus (HPV) in solitary adult laryngeal papillomas demonstrated by in-situ DNA hybridization with sulphonated probes. Human papilloma virus (HPV) types 6 and 11 have been repeatedly demonstrated in multiple laryngeal papillomas, and there is little doubt that these lesions are caused by HPV. It has been clearly demonstrated in recent reports that the clinical course of solitary adult onset laryngeal papillomas is entirely different from that of multiple papillomas of juvenile as well as of adult onset. We here report the presence of HPV types 6 and 11 in 19 out of 20 solitary papillomas from 16 patients, while HPV types 16 and 18 were totally absent. We conclude that the milder clinical course in such patients is most likely to be due to host factors, rather than to viral factors. 