Increased neopterin concentrations are observed in many diseases in which cellular immunity is stimulated. Gastroenteritis is still a major cause of childhood morbidity and mortality worldwide. Over 75% of all cases are caused by viral infections. Therefore a non-specific, quick, sensitive, cheap and easy-to-perform marker of viral infection, such as neopterin concentration in serum, would be a cost-effective method of identifying patients with gastroenteritis into probable viral etiology and therefore it would optimize treatment and reduce inappropriate antibiotic treatment.