The aim of this study was to evaluate the underlying lesions and factors predicting cervical intraepithelialneoplasia (CIN) 2+ in women who had “atypical squamous cells of undetermined significance” (ASC-US) oncervical cytology in the region with a high incidence of cervical cancer. This study was prospectively conductedat Chiang Mai University Hospital, Chiang Mai, Thailand. All women with ASC-US cytology undergoingcolposcopic evaluation between October 2004 and August 2008 were recruited. During the study period, 208women were enrolled. Mean age was 44.4 years. The histopathologic results at the initial evaluation were asfollows: CIN 2-3, 21 (10.1%); adenocarcinoma in situ, 3 (1.4%); cancer, 5 (2.4%); CIN 1, 26 (12.5%); and nolesions, 153 (73.6%). Multivariate analysis revealed that nulliparity (adjusted odds ratio [aOR] =4.09; 95%confidence interval [CI] = 1.04-16.10) and current oral contraceptive use (aOR=2.85; 95%CI= 1.14-7.15) wereindependent predictors for having CIN 2+ at the initial colposcopy. At the median follow-up time of 6.7 months,CIN 2-3 lesions were additionally detected in 2 women. In conclusion, ASC-US cytology in our population has arelatively high prevalence of underlying invasive carcinoma. Nulliparity and current oral contraceptive use areindependent predictors for harboring CIN 2+.