This is a report on a case of inborn methemoglobinemia. The disease has been diagnosed in the first week of infant's life. At first it was assumed it was a case of inborn heart defect because of expressed generalized cyanosis, but very soon it was excluded. Values of methemoglobin were extremely high, so three days after birth hereditary methemoglobinemia was diagnosed. Our diagnosis was confirmed by disappearing of cyanosis after ascorbic acid was applied, by values of methemoglobin reductase of the infant and its father, anamnestic data revealing that mother had had temporary cyanosis during her infancy, and satisfactory clinical course of illness.