Objectives-We previously reported that women polymorphic for the methylenetetrahydrofolate reductase (MTHFR) gene were less likely to have cervical intraepithelial neoplasia (CIN) 2 or 3 (odds ratio [OR] = 0.40, 95 % confidence interval [CI]: 0.21, 0.78, p=0.007). In the current study, we tested whether this protective association is modified by the circulating riboflavin status in the same study population.Methods-Riboflavin status was assessed by the erythrocyte glutathione reductase (EGR) assay and expressed in terms of an EGR activity coefficient (EGRAC). The status of MTHFR polymorphism, riboflavin, and circulating concentrations of folate, vitamins B-12, A, E, C and total carotene were ascertained in 170 White and 265 African-American women positive for the cervical presence of high-risk human papilloma virus (HR-HPV). Presence/absence of CIN 2 or 3 was determined histologically, and associations with risk factors were examined using multiple logistic regression. Eighty women with CIN 2 or 3 lesions were compared to 355 women without cervical lesions. Based on the median EGRAC of 1.4, women were grouped into low (> 1.4) and high (≤1.4) riboflavin status.Results-Women with MTHFR polymorphism and low riboflavin status were significantly less likely to have CIN 2 or 3 than the referent group of women without the polymorphism and high riboflavin status (OR = 0.35, 95% CI: 0.13, 0.92, p= 0.034). MTHFR polymorphism was not associated with CIN 2 or 3 in women with high riboflavin status (OR = 0.51, 95% CI: 0.22, 1.19, p = 0.119), nor were any of the associations influenced by folate levels.Conclusions-A further inactivation of polymorphic MTHFR by low riboflavin status and a resulting shift in the folate metabolic pathway toward DNA synthesis may explain these observations. The practical implications of this complex gene-nutrient-disease interaction will require further investigation.