The performance of thin-layer cervical cytology with the use of ThinPrep (Cytyc Corporation, Boxborough, MA) was assessed by comparing the original independent diagnosis of ThinPrep slides and conventional smears prepared from 1780 split samples with the most abnormal diagnosis per patient on the basis of an independent pathologist's masked review and with the detection of cancer-associated types of human papillomavirus (HPV) DNA. Cases were selected on the basis of the original diagnoses to include all discordant pairs (those diagnosed as atypical squamous cells of undetermined significance or higher grade, n = 1017), all concordant abnormal pairs (n = 444), and a random 5% of concordant normal pairs (n = 319). In screening centers, thin-layer cytology detected 135 (70.3%) of 192 women diagnosed as having squamous epithelial lesions or a higher grade in the independent review, whereas locally read smears detected 91 (47.4%) of these patients (P < .001). In hospital-based cytology laboratories, thin-layer cytology detected 308 (86.3%) of 357 women diagnosed with SILs or a higher grade in the independent review, compared with 283 (79.3%) diagnosed with smears (P = .011). Cancer-associated types of HPV DNA were detected in a slightly higher proportion of women with smears diagnosed as SILs than in women with thin-layer cytology diagnosed as SILs, whereas the overall number of HPV-associated SILs diagnosed was higher with thin-layer cytology. These data suggest that the ThinPrep method detects a higher percentage of SILs as defined in a masked, independent review than do concurrently prepared smears and that diagnoses of SILs rendered with ThinPrep correlate with the detection of cancer-associated types of HPV.