Objective: To determine risk factors of high-grade cervical intraepithelial neoplasia (CIN 2-3) among women with atypical squamous cells of undetermined significance (ASC-US) Papanicolaou (Pap) smears. Methods: Twohundred and sixty-six women with ASC-US Pap tests who underwent a colposcopy with histologic study were enrolled between August 2008 and June 2010. Patient data including age, education, income, parity, current pills used, number of vaginal deliveries, number of sexual partners, age at first sexual intercourse, history of sexually transmitted diseases, history of pelvic inflammatory disease and smoking habits were obtained. Logistic regression analysis was used to evaluate factors associated with CIN 2-3. Results: CIN was diagnosed in 134 of 266 women (50.4%). Ninty-seven of these (72.4%) had CIN 1 and 37 (27.6%) had CIN 2-3. Frequency of the latter was lower in women who had at least bachelor's degree graduation as compared to those having less than primary school graduation (odds ratio (OR) 0.085, 95% confidence interval (CI) 0.013-0.557). In addition, those with a higher income (5,000-20,000 baht per month) had less frequent CIN 2-3 (OR 0.378, 95% CI 0.147-0.970), whereas women who had 3 or more sexual partners were more likely to develop CIN 2-3 (OR 3.181, 95% CI 1.316-7.687). Conclusions: Women with ASC-US Pap smears who had 3 or more sexual partners, low education and low income were at an increased risk of CIN 2-3. Therefore, this group of patients deserve a high priority for immediate colposcopy in order for early detection of high-grade CIN.