Few studies have examined the association between maternal vitamin B-12 status and their breast-fed infants' findings. The objective of this study was to analyze the association of maternal B-12 status with infant findings including neurodevelopmental outcome in breast-fed babies with B-12 deficiency. Correlation analyses between the laboratory findings of infants with B-12 deficiency (n=120) and their mothers were performed and the association of maternal B-12 status with infant findings including the Denver-II developmental screening test (DDST II) results was examined. There was a significant correlation between infant and maternal B-12 levels (r=0.222; P=0.030), and between infant and maternal homocysteine (Hcy) levels (r=0.390; P<0.001). Among the babies 4 months of age or older, maternal Hcy levels were significantly correlated with infant mean corpuscular hemoglobin (r=0.404; P=0.001) and infant mean corpuscular volume (r=0.461; P<0.001). Mothers of infants with abnormal DDST II had lower vitamin B-12 (196.9 +/- 41.2 vs. 247.0 +/- 77.0 pg/mL; P=0.018) and higher Hcy levels (17.3 +/- 5.0 vs. 10.7 +/- 3.1 mu mol/L; P<0.001) than mothers of infants with normal DDST II. A lower maternal vitamin B-12 status may be related to impaired neurodevelopment in breast-fed infants with vitamin B-12 deficiency. Pregnant and lactating women should be offered easy access to healthy nutrition and vitamin B-12 supplements.