Selenium, as an essential micronutrient, is required for the proper functioning of the immune system and its deficiency affects the occurrence, virulence, or disease progression of some viral infections. We conducted a study to determine the serum selenium levels of children with acute gastroenteritis of possible viral origin and the effect of the serum selenium levels on the severity and the morbidity of the disease. The study was performed prospectively on 109 children aged 2-24 months with diarrhea of less than 8 days' duration admitted to the Diarrheal Disease Training and Treatment Unit. Blood samples were taken for selenium measurement on admission and 7-10 days after the end of the disease. Forty-three healthy children formed the control group. The mean serum selenium level on admission (62.41+/-13.06 mug/dl) was significantly lower than the mean of the second samples 7-10 days after the end of the diarrhea (81.73+/-17.10 mug/dl). The mean of the control group was 74.36+/-10.75 mug/dl, which was lower than the mean of the second samples but higher than the first sample. The frequency of vomiting and purging on admission and at the control visit, duration of diarrhea on admission, total duration of diarrhea, dehydration, breastfeeding, sex of the patients, and severity score of the disease did not alter the serum selenium levels. No correlation was detected between serum selenium levels and the parameters above. Further studies about the changes in selenium status during infectious diseases and the effect of selenium status on related mortality and morbidity are required to determine if there is need for supplementation.