The vitamin B12 statuses of Thai women with high- and low-grade cervical dysplasia were studied and compared with women with normal cytological smears. Serum vitamin B12 and vitamin B12 intakes were assessed, as well as demographic characteristics, sexual behavior, reproductive and menstrual history, exogenous hormone use, personal and familial medical history, smoking habit, and other risk factors. The presence or absence of genital HPV DNA was determined by polymerase chain reaction (PCR). Serum vitamin B12 levels in women with normal cytological smears were significantly higher than those with both high- and low-grade cervical dysplasia (p<0.001). Low vitamin B12 serum levels were significantly statistically associated with increased low-grade (OR = 4.08; 95% CI = 1.41-11.79; p < 0.05) and increased high-grade cervical dysplasia risk (OR = 3.53; 95% CI = 1.24-10.04; p< 0.05) for the highest vs lowest quartiles of serum vitamin B12. This study indicated a relationship between low vitamin B12 status and increased risk of cervical cancer.