Intravascular lymphomatosis is a rare disease, characterized by occlusion of small arterioles and veins by malignant lymphoma cells. It often affects the central and peripheral nervous system and the skin. This case report illustrates the diagnostic challenge. Most patients develop rapidly progressive dementia combined with focal neurological symptoms. No relevant clinical, radiological or laboratory tests are available to diagnose the disease. Blood samples and bone marrow are usually normal. Diagnosis is often delayed and made post mortem. Brain biopsy could be diagnostic. The prognosis is poor, but early aggressive chemotherapy can potentially prolong the life span and in a few cases be curative. Intravascular lymphomatosis should be considered in patients with a combination of rapidly developing encephalopathy and focal neurological signs.