Vitamin B12 status has been linked to cognitive impairment among older adults. Deficit in methylmalonic acid (MMA) may be reflective of cognitive impairment because it is a biochemically sensitive marker of B12 deficiency. In a cross-sectional study the contributions of different indices of B12 status, including serum B12, MMA and total homocysteine (tHcy), were measured in relation to cognitive functioning. B12 deficiency as measured by elevated MMA concentrations appeared to be most reflective of cognitive impairment and appeared to contribute unique variance to cognitive measures after controlling for other biochemical variables. Demographic variables, particularly education and age, were more strongly associated with cognitive measures than was MMA. Monitoring and reducing serum MMA concentrations by increasing the intake of vitamin B12 may provide protection against cognitive decline in this and other older populations.