BACKGROUND Atypical squamous cells of undetermined significance (ASCUS) is a cytopathologic term used to describe cases without specific pathologic substratum. Between 10‐60% of ASCUS cases correspond to squamous intraepithelial lesions (SIL). METHODS The objectives of this study were: 1) to detect the pathologic significance of ASCUS in study patients, 2) to determine whether PAPNET identifies these cases, and 3) to compare the results of PAPNET with those of a second conventional screening. One hundred and sixty‐three consecutive patients with the cytologic diagnosis of ASCUS and adequate follow‐up were selected. Of these, 111 patients had colposcopic lesions and biopsies were performed; in the remaining 52 cases colposcopy was negative, as were 3 consecutive annual Papanicolaou smears. In a blind review, all 163 cases were rescreened using PAPNET. A second manual screening was performed for comparison. RESULTS One hundred and twenty‐six of the 163 cases (77.3%) showed no SIL on biopsy or follow‐up. Of the 37 pathologic cases, the diagnosis was koilocytosis (flat condyloma) in 13 cases (8%), cervical intraepithelial neoplasia (CIN) type I in 11 cases (6.8%) low grade SIL [LSIL] in a total of 24 cases [14.8%]), and CIN II‐III or high grade SIL (HSIL) in 11 cases (6.8%). In the review using PAPNET, 57 previous ASCUS cases were classified correctly as negative, and 7 of 13 koilocytosis cases (54%), 9 of 11 CIN I cases (82%), and 7 of 11 CIN II‐III cases (64%) were diagnosed correctly. In the second conventional screening, 74 cases were negative and 77 cases were ASCUS; only 3 of 13 koilocytosis cases (23%), 4 of 11 CIN I cases (36.4%) and 5 of 11 CIN II‐III cases (45.5%) were reclassified correctly. CONCLUSIONS Among 163 patients with ASCUS, 77.3% had no precancerous squamous lesions. Concordance with definitive diagnosis was more accurate in our study using PAPNET analysis (Kappa index [K] = 0.7158) than by second conventional screening (K = 0.4537). Furthermore, we reclassified 35% of smears as negative and 15% as SIL. Cancer (Cancer Cytopathol) 1998;84:361‐365. © 1998 American Cancer Society.