Methemoglobinemia and mental retardation associated with NADH-diaphorase deficiency was found in a 2-year-old girl of Spanish origin. She showed no NADH-diaphorase activity in either erythrocytes or leukocytes, but electrophoretic studies of the hemolysate showed traces of an enzyme with normal mobility. Cytochrome b5 reductase activity was also found to be absent in the leukocytes of the propositus. Intermediate NADH-diaphorase activity was found in erythrocytes and leukocytes in her parents and her sister in accordance with the autoso-mal recessive mode of inheritance of this enzymopathy. The relationship between a generalized cytochrome b5 reductase deficiency and the progressive neurological involvement in our patient is discussed briefly.