Early antiviral treatment is important for herpes simplex encephalitis. A reliable test for supporting empirical therapy with antiviral agents is urgently needed. Sixty-six children with fever, focal seizures, and consciousness disturbance had their sera examined for anti-herpes simplex viral immunoglobulin M (IgM) antibody with enzyme-linked immunosorbent assay. The result was positive in all seven patients confirmed to have herpes simplex encephalitis. Only two of the other 59 patients were positive on this test. The average period for the development of serologic positivity was 7.3 days from the onset of neurologic symptoms. Although most sera were not tested in the early stage, all were collected on admission day. Our data suggest that before further evidence indicates another diagnosis, this simple, but specific IgM antibody test could be used as a guide for deciding to continue the antiviral treatment for serologically positive cases or to discontinue antiviral treatment for serologically negative ones. Yet, we would not suggest diagnosing herpes simplex encephalitis with this test alone.