Pyogenic ventriculitis is an uncommon and potentially fatal central nervous system infection. Delayed treatment due to non specific clinical symptoms may lead to an unfavorable outcome. Brain magnetic resonance imaging (MRI) plays an important role in the diagnosis of pyogenic ventriculitis. We describe two patients with pyogenic ventriculitis presenting with a pathognomonic MRI finding. The first patient, a 77-year-old female, developed high fever and consciousness disturbance. MR images revealed hyperintense lesions with a fluid-fluid level in the bilateral lateral ventricles on diffusion-weighted images (DWIs) and hypointense lesions on T2-weighted images (T2WIs). MR images also revealed findings of left otitis media. The second patient, a 63-year-old male, who had a past history of multiple myeloma and had received chemotherapy, developed high fever and left hemiparesis. MR images revealed a hyperintense lesion with a fluid-fluid level in the right lateral ventricle on DWIs and a hypointense lesion on T2WIs, multiple ring-enhancing lesions on gadolinium enhanced T1-weighted images, and pontine infarction on DWIs. Chest computed tomography revealed an infiltrative shadow in the lower lobe of the left lung. On the basis of MRI findings, both patients were diagnosed as having pyogenic ventriculitis and were administered high-dose meropenem intravenously. The second patient was also administered sulfamethoxazole-trimethoprim orally. Intraventricular abnormalities disappeared and the patients achieved complete remission after the antibacterial treatment. Intraventricular hyperintense lesions on DWIs and hypointense ones on T2WIs with a fluid-fluid level is a pathognomonic finding of pyogenic ventriculitis and has not been previously reported in other diseases. Recognition of the characteristic MRI features and initiation of high-dose and appropriate antibiotics in an early stage may lead to a favorable outcome of the disease.