Risk factors for cervical squamous intraepithelial lesions (SIL) including human papillomavirus (HPV) infection and the p53 codon 72 polymorphism were investigated in a case-control study with 103 cases and 105 controls in Northeastern Thailand. Increased risk for SIL was observed for age at menarche (odds ratio (OR) = 2.2; p< 0.005), age at the first sexual intercourse (OR=2.4; p< 0.05), number of sexual partners (OR=2.7; p< 0.005) and partners' smoking history (OR=2.3-3.2; p< 0.01). Prevalence of malignant type of HPV infection in the control and SIL groups was 18.1% and 60.2%, respectively. HPV infection significantly increased risk for SIL 6.8-fold (p< 0.001). HPV-16 infection was the commonest (31 out of 62 carriers) in SIL patients and highly associated with risk. The p53 codon 72 polymorphism was not identified as a genetic risk for SIL in this study, as demonstrated in Thai cervical cancer. Therefore, to prevent cervical neoplasia or HPV infection, inclusion of knowledge on sexual behavior and effects of smoking into public health programs is important and, at the same time, a nation-wide screening scheme for cervical abnormalities including HPV-typing is a high priority in Thailand.