A rare type of diffuse large B-cell lymphoma, intravascular large B-cell lymphoma primarily affects the middle-aged to elderly population, with a slight predominance in men. By the time of presentation, most patients have advanced, disseminated disease, and often the diagnosis is made at autopsy. Patients may present with any of a myriad of symptoms, with any tissue potentially being infiltrated. Central nervous system and cutaneous involvement is common, as is the presence of B symptoms including fever, weight loss, and night sweats. Morphologically, growth of neoplastic cells is restricted to the lumen of small vessels. The cells are large, with 1 or more prominent nucleoli, scant cytoplasm, and frequent mitotic figures, and are commonly positive for cluster of differentiation markers 79a, 20, and 19, as well as MUM1/IRF4 and Bcl-2. Intravascular large B-cell lymphoma is aggressive, and without treatment is rapidly fatal.