Human papillomavirus (HPV) DNA was detected by Southern blot hybridization in cervicovaginal lavage samples from 199 of 329 (60.5%) women attending a municipal hospital colposcopy clinic. Human papillomavirus was identified in 195 of 264 (73.9%) patients with a squamous intraepithelial lesion or cancer on biopsy or Papanicolaou smear (Bethesda system) compared with 11 of 65 (16.9%) without squamous intraepithelial lesion (P < .0001). The most common HPV type identified was HPV 16 (20.6% of positive samples), and 36.7% of isolates contained uncharacterized HPVs. Of women with cervical intraepithelial neoplasia (CIN) grade III or cancer, 23.4% were infected with HPV 16 compared with less than 4% with any other single HPV type. Based on biopsy diagnosis in patients infected with specific HPV types, HPVs 6 and 11 had low oncogenic potential; HPVs 18, 31, 35, and 45 had intermediate oncogenic potential; and HPVs 16 and 33 had high oncogenic potential. Hyperchromatic, unusually enlarged nuclei ("meganuclei"), and/or abnormal mitoses were found significantly more often in lesions infected with HPVs 16, 33, and 35 than in those infected with HPVs 6, 11, 18, 31, and 45, even in low-grade lesions, and may represent a histologic marker for HPVs with significant oncogenic potential. Human papillomavirus capsid protein was detected significantly less often by immunocytochemical staining in CIN I and CIN II lesions infected with HPVs 16 and 33 (8.3%) than in those infected with HPVs 6, 11, 18, and 31 (60%; P = .007), suggesting early abnormalities in cellular differentiation in lesions infected with highly oncogenic HPVs.