Methionine-dependence phenotype (MDP) refers to the reduced ability of cells to proliferate when methionine is restricted and/or replaced by its immediate precursor homocysteine. MDP is a characteristic of human tumors in vivo, human tumor cell lines, and normal somatic tissue in some individuals. It was hypothesized that MDP is a risk factor for developing breast cancer in BRCA (BRCA1 and BRCA2) germline mutation carriers. To test the hypothesis, human peripheral blood lymphocytes of BRCA carriers with and without breast cancer and healthy non-carrier relatives (controls) were cultured for 9 days in medium containing either 0.1 mmol/L L-methionine or 0.2 mmol/L D,L-homocysteine, with the ratio of viable cell growth in both types of medium after 9 days used to calculate the methionine-dependence index (MDI), a measure of MDP. We also tested whether MDP was associated with common polymorphisms in methionine metabolism. Viable cell growth, MDI, and polymorphism frequency in MTRR (A66G and C524T) and MTHFR (A1298C and A1793G) did not differ among the study groups; however, MDI tended to be higher in BRCA carriers with breast cancer than those without and was significantly increased in MTHFR 677T allele carriers relative to wild-type carriers (P=0.017). The presence of MTR A2756G mutant allele and MTHFR C677T mutant allele in carriers was associated with increased breast cancer risk [odds ration, 3.2 (P=0.16; 95% confidence interval, 0.76-13.9) and 3.9 (P=0.09; 95% confidence interval, 0.93-16.3), respectively]. The results of this study support the hypothesis that defects in methionine metabolism may be associated with breast cancer risk in BRCA carriers.