Background Cerebral venous thrombosis (CVT) is a distinct disorder with highly variable symptoms and courses. Malignancy is known to be one of the predisposing factors of CVT. Case Report We present the case of a 69-year-old man with sudden behavioral changes. Brain MRI, magnetic resonance venography, and positive D-dimer assay confirmed the presence of CVT. The work-up for detecting the cause of the CVT revealed occult prostate cancer. Conclusions Occult malignancy including prostate cancer should be strongly suspected in older patients with idiopathic CVT.