We report the clinical trial studies for the ThinPrep Imaging System (TIS; Cytyc, Boxborough, MA). Between December 2000 and July 2001, 10,742 ThinPrep specimens were collected at 4 US clinical sites representative of the normal clinical population of the laboratories, including screening patients and referred patients. After nonstudy screening diagnoses were completed, the vials were relabeled and randomized, and study slides were prepared and stained. TIS-trained cytotechnologists and pathologists screened the slides twice, first manually, then TIS-assisted after an appropriate interval. Afterward, 3 independent pathologists performed an adjudication study to determine definitive diagnoses for the nonnegative slides and 5% of the negative slides; the adjudicated diagnoses served as the "gold standard" for subsequent sensitivity and specificity analyses. TIS-assisted screening was statistically more sensitive than manual screening for atypical squamous cells of undetermined significance (ASCUS) or higher (+) and statistically equivalent for low- (LSIL)+ and high-grade squamous intraepithelial lesion (HSIL)+ diagnoses. TIS-assisted screening had equivalent specificity for ASCUS+ and LSIL+ and significantly higher specificity for HSIL+. Average cytologists' daily screening rates doubled with TIS-assisted screening. The sensitivity of the TIS-assisted screening system equals or exceeds the sensitivity of manual primary screening without adversely affecting specificity, and TIS-assisted screening can improve cervical cancer screening productivity. Cost issues require further study.