Increased urinary methylmalonic acid (UMMA) concentrations might indicate vitamin B-12 deficiency. Our study tested the hypothesis that elevated UMMA in breast-fed infants is associated with decreased maternal serum B-12 concentrations. UMMA concentrations were measured in 17 vegetarian mothers and their infants and in six infants of nonvegetarian mothers. Serum vitamin B-12 concentrations were determined in all mothers. Range of UMMA for vegetarian infants (3-924 mcg/mg [2.6-790.9 mumol/mmol] creatinine) was much broader than that for omnivorous infants (2-25 mcg/mg [1.7-21.4 mumol/mmol] creatinine). Maternal UMMA and serum vitamin B-12 were negatively correlated (r = -0.700, p = 0.003). Infant UMMA concentrations correlated positively with maternal UMMA concentrations (r = 0.686, p = 0.003) and inversely with maternal serum vitamin B-12 concentrations (r = -0.681, p less than 0.001). In three infants with high UMMA concentrations, vitamin B-12 treatment (oral B-12, vitamin B-12 injection, or a modification of maternal diet within the vegetarian philosophy) led to an abrupt decrease of UMMA.