Conditions with cobalamin deficiency are common and may represent a considerable diagnostic challenge. This deficiency state may result in a wide range of uncharacteristic symptoms, and the traditional laboratory assays may give inconclusive or even misleading results. This article briefly reviews the biochemistry, normal homeostasis, pathology, symptoms and signs resulting from cobalamin deficiency. Homocysteine and methylmalonyl-CoA are substrates in the two cobalamin-dependent pathways. Recent research suggests that measurements of homocysteine and methylmalonic acid in blood may constitute a simple, sensitive, and specific tool in the diagnosis and follow-up of the cobalamin-deficient patient.