Aqueductal stenosis can result from mumps meningoencephalitis. We report a case of acute hydrocephalus from an aqueductal stenosis developed immediately after a clinically evident mumps infection. The patient, aged two years, was treated with an antiedema therapy, and a ventriculoperitoneal shunt was placed. His clinical conditions rapidly improved. Only mumps virus was found in his cerebrospinal fluid culture. This case probably represents the earliest hydrocephalus from mumps, among those reported in the literature.