There is concern that exposure of patients to folic acid may prevent the development of the macrocytosis of cobalamin deficiency and thus delay the detection of the neurological complications. We examined the relationship between low cobalamin levels and mean cell volume (MCV) at different serum folate concentrations in 63,472 blood samples tested in a community pathology laboratory over 2 years. We found no evidence that high serum folate levels masked the macrocytosis of cobalamin deficiency in this population with similar increases in MCV regardless of whether the serum folate was low, normal or high. Macrocytosis appears to retain its value as a marker of cobalamin deficiency in people with serum folate concentrations above the population average.