The prevalence of clinically significant methemoglobinemia (MHb) is unknown among previously healthy infants presenting to the emergency department (ED) with diarrhea and dehydration. However, it is well described in these infants and, when present, can be associated with significant morbidity [1-4]. Nevertheless, it may not be clinically apparent until levels reach almost 7 times normal levels [5]. Based on this, we hypothesized that dehydrated infants younger than 3 months with diarrhea would commonly have MHb levels above normal but below that causing cyanosis and obvious distress.