Recessively inherited NADH-cytochrome B5 reductase deficiency, when present in the homozygous or doubly heterozygous form, is manifested by two different clinical presentations, depending on the nature and cellular distribution of the mutant enzyme. The observations supporting a clinical and biochemical classification of enzymopenic hereditary methemoglobinemia are summarized. Type I, with deficiency demonstrable only in the erythrocytes, presents as uncomplicated, benign methemoglobinemia. Type II, generalized cytochrome B5 deficiency demonstrable in all of the tissues that have been examined, is accompanied by severe, lethal, progressive neurological disability, in addition to methemoglobinemia. Type III deficiency is limited to hematopoietic cells and resembles Type I clinically. Type IV, also clinically like Type I, is associated with deficiency of the cofactor, cytochrome B5. Except for Type IV, the different types appear to be the result of mutations in paired alleles of a gene on chromosome 22 that affect the catalytic activity or stability of the cytochrome B5 reductase.