A recent Cochrane review reported that although vitamin B-12 deficiency is known to be associated with cognitive impairment in old age, benefits of supplementation on mental ability are unclear. The situation is similar to that for blood pressure, in which hypertension is associated with neuropsychological deficits in adulthood but cognitive outcomes of lowering blood pressure in randomised controlled trials are equivocal. We found that for blood pressure, the limited effect of intervention is partly explained by the relation between childhood IQ and adult blood pressure—children with higher IQs had lower mid-life blood pressures. As approximately 50% of the variance in adult IQ is explained by childhood IQ, studies failing to account for ‘‘pre-morbid’’ mental ability are likely to overestimate the association between adult IQ and blood pressure. Previously we reported significant relations between various tests ofmental ability in old age and vitamin B- 12 and folic acid concentrations in the blood. We now describe the association between vitamin B-12 and serum folate and lifetime change in mental ability using the same cognitive test at age 11 and age 79.