Using The Bethesda System, five pathologists independently diagnosed 200 smears that originally had been classified as "atypical," and the results were correlated with concurrent detection of human papillomavirus (HPV) DNA by Southern analysis and by polymerase chain reaction amplification. The smears were reclassified as benign reactive changes (negative), atypical squamous cells of undetermined significance, or squamous intraepithelial lesion (SIL). Exact five-way cytologic agreement was achieved in only 29% of smears, and no slide was diagnosed as atypical squamous cells of undetermined significance by all reviewers. The detection of high-risk types of HPV correlated strongly with the likelihood of a diagnosis of squamous intraepithelial lesion. High-risk HPV types were detected in approximately 60% of smears reclassified as squamous intraepithelial lesion compared with 30% of those reclassified as atypical squamous cells of undetermined significance and 10% of negative smears (P < .001). Every smear unanimously diagnosed by the panel as squamous intraepithelial lesion was associated with detectable HPV DNA, mainly of high-risk types. Low-risk HPV DNA types were found with similar frequency in all diagnostic categories assigned by the reviewers. Based on the consistent relation between high-risk HPV detection and diagnoses according to the Bethesda System, the authors conclude that HPV testing may have an important role in quality assurance in cervical cytopathology.