Abstract: Objective 　To investigate the role of folate deficiency and MTHFR C677 T polymorphism on genetic damage in human lymphocytes f rom breast cancer cases and cont rols. Methods 　Lymphocytes with different MTHFR C677 T genotypes f rom the donors were cultured for 9 days in media with differ2 ent concent rations of folic acid (FA) and the f requencies of micronucleated binucleated cell (MNBN) was evaluated by cytokinesis2block micronucleus cytome assay (CBMN cyt) . Results 　The f requencies of mi2 cronucleated binucleated cell (MNBN) in 30 nmol/ L FA were significantly higher than those in 60, 120, 240, 2 260 nmol/ L FA ( P < 0. 001～0. 05), but no significant differences were observed either between those in 60 and 120 nmol/ L FA, or between those in 120, 240 and 2 260 nmol/ L FA within either groups when the genotype was the same. Within either of cancer case and cont rol group s, the f requencies of MNBN of mutant homozygotes ( TT) was significantly higher than that of wild isozygotes (CC) ( P <0. 01～0. 05) in any FA concent ration group. Af ter the genetic damage background of sample was re2 duced, there was no significantly difference in MNBN f requency between lymphocytes with the same gen2 otype f rom case and cont rol at the same FA concent ration. Conclusion 　FA concent ration below 120 nmol/ L increased genetic damage in human lymphocytes in vit ro. Individuals with MTHFR 677 TT geno2 type may be more sensitive to folic acid deficiency regardless the cancer statues than those with MTHFR 677CC and 677 CT genotypes.