To differentiate bacterial from viral infections the level of C-reactive protein in serum samples was studied in three groups of children under 5 years of age with gastroenteritis. Of the 53 children with bacterial infection, 41 (77%) had C-reactive protein levels > or = 12 mg1 -1, 32 (66%) > or = 20 mgl-1 and 24 (45%) > or = 35 mgl-1. Of the 35 patients with viral infection, 4 (11%) had C-reactive protein levels > or = 12 mgl-1, 3 (9%) > or = 20 mgl-1 and 1 (3%) > or = 35 mgl-1. The best balance between sensitivity and specificity of C-reactive protein was obtained for a cut-off level > or = 12 mgl-1 (sensitivity 77%, specificity 89%) as compared to > or = 20 mgl-1 (sensitivity 58%, specificity 97%) and > or = 35 mgl-1 (sensitivity 44%, specificity 97%). Our results suggest that the determination of C-reactive protein values may be a useful tool for predicting bacterial gastroenteritis in children.