Chronic lymphocytic leukemia (CLL) is the most common leukemia in many countries. Infections are the most common causes of morbidity and mortality; lymphoid cell dysfunction and neutropenia associated with chemotherapy are main predisposing conditions for infection. For this reason infectious conditions must be excluded in a case with CLL and fever. Richter's transformation (RT) is a kind of lymphoma that is a rare condition in CLL cases Fever of unknown origin is a rare finding in Richter's transformation (RT) but it has been reported as anecdotal reports. Here a case with RT as the cause of fever in a case with CLL was reported and literature was reviewed.