The authors have demonstrated the presence of human papillomavirus (HPV) types 6 and 11 in 10 of 13 (77%) juvenile laryngeal papillomatosis by in situ DNA hybridization using as probes the radiolabeled DNAs of HPVs 6, 11, 16, and 18. Of six specimens from adult laryngeal papillomatosis assayed by the same technique, only 33% were positive. Immunohistochemistry to detect HPV capsid antigens performed on serial sections gave positive signals in 44% (8 of 18) of the specimens, all from juvenile lesions. These results were in agreement with in situ hybridization, except in two cases. When both series (juvenile and adult) were analysed by amplification of a 450-bp fragment corresponding to the L1 ORF of the HPV genomes directed by the polymerase chain reaction, the frequency of positive specimens rose to 100%. Our data agree with the concept that HPV is implicated in the etiology of laryngeal papillomatosis.