Intravascular large B-cell lymphoma (IVLBCL) is a rare subtype of non-Hodgkin's lymphoma (NHL) and that progresses rapidly and is usually fatal. Because it usually presents with nonspecific symptoms, such as fever, the early diagnosis of IVLBCL is very difficult and it is often misdiagnosed as another disease. Systemic lupus erythematosus (SLE) is an autoimmune disease that affects various organs. The clinical manifestation of SLE ranges from rash and arthritis through anemia and thrombocytopenia to serositis, nephritis, seizures, and psychosis. Thus, it can be easily confused with many other disorders. We report a case of IVLBCL mimicking SLE in the initial diagnosis.