The aim of this study was to determine the relationship between red cell folate levels and the presence of cervical intraepithelial neoplasia (CIN) or cervical human papillomavirus infection (HPV) (or both). For that purpose, we designed a case-control study.One hundred three asymptomatic women who were between the ages of 20 and 76 and attending the gynecological clinic of the Military Hospital in Bogota, Colombia, were selected. Their mean age was 37.9 years. Inclusion criteria combined a colposcopic examination and a cervicovaginal Papanicolaou smear; accordingly, patients were divided into women without CIN or HPV (55 women, the control group) and women with CIN or HPV (or both) (48 women, the study group). Red cell folate levels were determined by radioimmunoassay.Statistically significant differences in folate levels were found between cases [2.55 ng/ml; standard deviation, 0.75; 95% confidence interval, 2.34-2.73] and controls (2.96 ng/ml; standard deviation, 0.77; 95% confidence interval, 2.75-3.14; p < .008). The odds ratio between the folate levels and CIN or HPV (or both) was 0.49 (p = .01). Red cell folate levels of the individuals participating in this study were not found to be associated with parity, the use of oral contraceptives, cigarette smoking, or age.High red cell folate levels appear to provide a protective effect against the development of CIN or HPV (or both).