Although genital human papillomavirus (HPV) infection is well established as the etiologic agent for cervical intraepithelial neoplasia (CIN), little is known about the cofactors involved in the development of high-grade lesions or the progression of low-grade to high-grade lesions. In our study of HPV-infected women with CIN (163 CIN I, 51 CIN II and 44 CIN III), women with CIN II or III were compared with those with CIN I for risk factors associated with high-grade lesions. After controlling for age, education, ethnicity and frequency of Pap smear screening, infection with HPV 16, but not high viral load or infection with multiple types, was associated with high-grade lesions (OR for CIN II = 11.96, OR for CIN III = 23.74). Risk of CIN III, but not CIN II, increased with number of cigarettes smoked per day (ORs = 1.49 and 3.35 for < or = 10 and > 10 cigarettes per day, respectively) and decreased with frequency of condom use during sex (ORs = 0.60 and 0.32 for women who used condoms occasionally/sometimes and most/all of the time, respectively). There were no associations between high-grade lesions and plasma levels of micronutrients (retinol, beta-carotene, alpha-tocopherol and reduced ascorbic acid). Our results indicate that infection with HPV 16 is associated with high-grade lesions. Additional cofactors, such as cigarette smoking, may be required as a carcinogen to advance HPV-infected cells toward neoplastic progression.