Objective: To determine the prevalence of cervical intraepithelial neoplasia (CIN) 2-3 (high-grade CIN) among women with Atypical Squamous Cells of Undetermined Significance (ASC-US) Pap smear. Material and Method: A retrospective medical record review of 220 women with ASC-US cervical Pap smear, including age, menstruation status, parity, placed of residence, occupation, main complaint, and definite histopathological result between July 2007 and January 2010 was done. Results: The prevalence of high-grade CIN 2 and CIN 3 in Thammasat University Hospital were 8.6% and 3.2% respectively. No cancer was found in the present study. There was no statistically significant difference in the prevalence of high-grade CIN between the patients who were 50 years old or more and those who were younger. Conclusion: Prevalence of high-grade CIN in women with ASC-US was 11.8%. Immediate colposcopy is recommended in women with ASC-US.