A serum cobalamin level lower than 250 pmol/l combined with high values of plasma homocysteine and serum methylmalonic acid confirms the diagnosis of vitamin B12 deficiency. A low serum and erythrocyte folate level and high plasma homocysteine confirm folate deficiency. If the cobalamin level is higher than 250 pmol/l no further tests are needed. In patients with neurologic or psychiatric disorders combined with elevated levels of homocysteine and methylmalonic acid, cobalamin deficiency is likely if these levels decrease during vitamin B12 therapy. The causes of elevated homocysteine levels are often obscure, and homocysteine should therefore not be used as a screening test.