Malignant external otitis (MEO) is a severe infection of the external auditory canal and skull base which most often affects elderly patients with diabetes mellitus. Early diagnosis and aggressive treatment are important in terms of reducing risks of dissemination. We report four cases of MEO in patients with diabetes mellitus who were observed with computed tomography (CT) and/or magnetic resonance imaging (MRI). MRI was superior to CT in patients with MEO in terms of estimating the anatomic extent of the disease, but it cannot be used for monitoring therapy.