A 14-year-old girl presented with symptoms and signs of left otomastoiditis and meningitis. CT and MRI revealed a largely expanded cavum septum pellucidum with enhancing walls and findings suggesting left otomastoiditis and meningitis. At surgery, an abscess was found within the cavum septum pellucidum. Five similar cases have been reported in the literature. To our knowledge, this is the first case associated with otomastoiditis. Although rare, abscess of the cavum septum pellucidum should be suspected in patients with meningitis and/or head trauma and delayed clinical deterioration.