Febrile children are often given antibiotics empirically and unnecessarily. MxA is a protein induced in peripheral lymphoid cells by type 1 interferons during active viral infection. The ability of a whole blood ELISA assay for MxA to identify children with viral illness was studied in 122 children who presented with acute onset fever and 52 age-matched healthy controls. The febrile children were divided into three groups according to their final diagnoses: etiologically diagnosed viral infection, clinically diagnosed viral infection, and bacterial infection. MxA levels in the bacterial infection group and controls were similar and low (90.9 +/- 69.7 and 76.9 +/- 63.2 ng/mL, respectively). In contrast, mean MxA levels in the two viral infection groups were higher than in both the bacterial and control groups (719.2 +/- 386.4 and 827.0 +/- 651.1, respectively). A receiver operating characteristic analysis showed that the area under the curve of the MxA level was greater than under the curves of both the white blood cell count and the C-reactive protein concentration. Whole blood assay of MxA is a clinically useful tool for diagnosing viral illness in febrile children and should help reduce use of unnecessary antibiotics.