Inherited disorders of vitamin B12 include those which involve the inability of the vitamin to be absorbed from the gut and transported to the appropriate tissues, and those in which the vitamin is not utilised by target cells. The former include intrinsic factor abnormalities, selective malabsorption of vitamin B12 with proteinuria, and deficiencies of transcobalamin I and transcobalamin II. The latter include a defect in the release of free vitamin B12 from lysosomes (cblF), and defects in the formation of both vitamin B12 cofactors (cblC, cblD) or of adenosyl-B12 (cblA, cblB) or methyl-B12 alone (cblE, CblE variant). This article reviews the major clinical manifestations of these diseases, and provides an approach to the diagnosis of transcobalamin II deficiency and the cbl mutations using cultured cells.