Recently, incidence of acute mastoiditis decreased markedly, but we experienced five cases of acute mastoiditis over the last three years. The five, patients presented with various clinical courses; findings, symptoms, and/or treatments. The diagnosis of the acute mastoiditis was made based on individual findings and symptoms. Especially, computed tomography of mastoid could play a reliable role to make the diagnosis. Acute mastoiditis with cholesteatoma should be treated more carefully, because cholesteatoma could often extended to the intracranial area. However, acute mastoiditis was not necessarily treated by surgical intervention in the first choice. Five patients in this report had different treatments including conservative tretment according to each clinical course.