Methemoglobinemia is a serious medical condition that affects hundreds of infants in the United States each year. The condition involves the oxidation of red cell hemoglobin to a state that is unable to transport oxygen. Affected infants appear cyanotic and may have altered mental status. The condition is readily reversible if recognized and treated appropriately. The Wisconsin Division of Public Health investigates all cases of infant methemoglobinemia in an attempt to determine their cause. Between January 1990 and September 1999, 8 infants were diagnosed with this condition. Review of their hospitalization records found that 3 of these cases involved infants whose formula was prepared with water from nitrate-contaminated wels. Risk factors identified in the remaining cases included use of folk remedies, misuse of over-the-counter analgesics, and an inherited enzyme deficiency. Causes were not identified for 2 of the cases. All of the affected infants recovered.