Risk factors, cytologic and histopathologic features, and human papillomavirus (HPV) detection associated with 75 cervical smears classified as atypical squamous cells of undetermined significance, rule out high-grade squamous intraepithelial lesion (ASCUS, rule out HSIL) were reviewed. Cases were identified in a pathology panel review of material collected from 1953 women participating in a 5-year prospective study of HPV infection and squamous intraepithelial lesions at Kaiser Permanente, Portland, Oregon, sponsored by the National Cancer Institute. Initial abnormal smears diagnosed as ASCUS, rule out HSIL by one panelist or diagnosed as ASCUS by one pathologist and as HSIL by another were included. The 75 ASCUS, rule out HSIL smears identified were examined again by two pathologists after the study. These cases were compared with cases of ASCUS, not otherwise specified (ASCUS, NOS) and HSIL identified in the same group of 1953 women. Findings in ASCUS, rule out HSIL included tissue fragments (21%); atypical immature metaplasia (17%); atypical mature metaplasia (15%); small atypical cells (9%); and atypical repair (4%). A final patient classification of HSIL, reflecting all available data, was assigned to 11 (24%) of 46 women with ASCUS, rule out HSIL and to 1(1%) of 80 women with ASCUS, NOS in the original review (P < .001). Detection of oncogenic HPV types at diagnosis in ASCUS, rule out HSIL; ASCUS, NOS; and HSIL was similar, but data were unavailable for many subjects. Among women not tested at diagnosis, enrollment testing (1 to 4 years earlier) revealed that HPV detection in women with ASCUS, rule out HSIL was intermediate in frequency between ASCUS, NOS and HSIL. These data suggest that ASCUS, rule out HSIL is a distinct diagnosis from ASCUS, NOS because it is more often associated with an underlying HSIL. Consequently, women with ASCUS, rule out HSIL should be referred for colposcopic examination.