We evaluated the impact of the Bethesda System (TBS) 2001 in cytology reporting of atypical squamous cells (ASC) when using conventional Pap smears and liquid-based cytology preparations (LBC). Follow-up information for all ASC cases encountered in Queen Mary Hospital, Hong Kong, China, from July 2000 to June 2004 (using TBS 1991 in the first 2-year period and TBS 2001 in the second) was analyzed. Among 4, 089 ASC cases studied, more than 50% had negative follow-up; this percentage was lower with TBS 2001. The percentage of ASC cases with a low-grade cervical intraepithelial neoplasia (CIN) end point was higher with TBS 2001, especially after application of LBC. There was also a decreasing trend of having this low-grade CIN end point with advancing age. Most clinically significant outcomes occurred after 6 months and before 1 year post-ASC diagnosis. With TBS 2001, more than 50% of ASC, cannot exclude high-grade squamous intraepithelial lesion (ASC-H) were associated with high-grade CIN on follow-up. TBS 2001 improves the positive predictive value of ASC for clinically significant lesions. Introduction of dichotomous subcategorization of ASC is relevant, with ASC-H associated with a much higher risk of subsequent high-grade squamous lesions.