More than 22 types of human papillomavirus (HPV) have been detected in genital tract squamous cell intraepithelial lesions. Seven of two hundred eighty-six (2.4%) genital tract tissues in which HPV DNA was detected by in situ hybridization contained two or more different HPV types. When analyzed by site, 5 of 204 (2.4%) of cervical intraepithelial lesions were infected by more than one type, compared with 2 of 82 (2.4%) of vulvar lesions. The rate for low-grade lesions was similar (5/218; 2.3%) to that for high-grade lesions (2/68; 2.9%). In contrast, two different HPV types were detected in 6/33 (18%) of tissues by the polymerase chain reaction (PCR) using type-specific primers for eight HPV types. It is concluded that infection by one HPV type is rarely associated with concurrent 'active' infection by a second HPV type, even though DNA of a different viral type can be detected by PCR in about one fifth of such cases. Further study is required to determine if an existing HPV infection can inhibit replication by a different HPV type.