PURPOSE: Methemoglobinemia has been observed during a diarrheal illness among those who were not exposed to oxidizing agents in neonates and young infants. We performed the study to determine predisposing factors in this entity. METHODS: The medical records of 24 patients were reviewed, who were admitted for diarrhea with methemoglobinemia to Ewha Womans University MokDong Hospital, from February 1994 to November 2000. The concentration of methemoglobin was measured by CO-oximeter(ABL520(R) , Radiometer, Sweden). RESULTS: All the patients were before 2 months of age, had poor weight gain and dehydration associated with diarrhea. One patient was a breast feeding infant, 20 were bottle feeding, and 3 were mixed feeding. The seasonal variation in the incidence of the disease was indistinct but more cases were found during the winter months. We observed cyanotic skin discoloration in 21 patients, vomiting in 13, lethargy in 13, fever in 9, decreased urine output in 8, tachypnea in 4, and irritability in 2. The mean concentration of methemoglobin was 19.8+/-14.6%(range 3.8-65.0%) on admisssion. The severity of methemoglobinemia was significantly correlated with blood WBC (r=0.506, P=0.012), arterial blood pH(r=-0.600, P=0.002), total CO2 concentration(r=-0.456, P=0.025) and base excess(r=-0.429, P=0.036). The underlying diseases were milk allergy(8 patients), sepsis (2), urinary tract infection(1), salmonellosis(1), viral meningitis(1) and unknown(11). The majority of patients were treated conservatively and 9 patients(37.5%) were treated with intravenous administration of methylene blue. CONCLUSION: This study suggested that diarrhea, acidosis, infection and dehydration may be certain predisposing roles for developing methemoglobinemia in neonates and young infants. We should consider checking methemoglobin concentration in patients with diarrhea and cyanosis.