The aim of the study was to identify immediate clinical and/or laboratory findings able to differentiate bacterial from viral etiology of acute gastroenteritis in pediatric patients. We studied 52 children, aged between 5 months and 12 years, consecutively admitted to hospital with acute diarrhoea lasting less than 5 days. All the patients were divided into 4 groups according to etiologic agent, subsequently demonstrated by culture: salmonellae (group A), rotavirus (group B); combined salmonellae and rotavirus (group C) and no pathogen (group D). The contemporary presence of fever > 39 degrees C, number of daily liquid stools > 6, presence of bloody diarrhoea, positivity of C-reactive protein and hyponatremia (< 135 mEq/l) allowed to recognize the etiology (viral or bacterial) before results of culture (sensitivity was 71% and specificity was 97%). In particular, hyponatremia resulted significantly lower in group A and C than in group B and D. We concluded that hyponatremia can be considered a marker for acute gastroenteritis caused by salmonellae.