Non-Hodgkin's lymphomas (NHL), Hodgkin's Disease (HD), and multiple myeloma (MM) develop as second malignancies in approximately 3%, 0.5%, and 0.1%, respectively, of chronic lymphocytic leukemia (CLL) patients. The true incidence may be higher, as postmortem examination is not performed in most patients, thus underestimating occult disease. As originally described, the term Richter's syndrome (RS) refers to the development of aggressive NHL during the course of CLL. The onset of RS is usually abrupt with clinical deterioration as manifested by worsening systemic symptoms, rapid tumor growth, and/or extranodal involvement. Diagnosis requires tissue biopsy. The NHL is usually diffuse large cell (LCL) or its immunoblastic variant. It is resistant to current therapies, and the median survival of patients who develop RS is approximately 6 months. The precise relationship between the cells of origin of CLL and LCL in RS patients is unknown with data suggesting either common (60%) or distinct (40%) clonal evolutions for these malignancies in different patients. Gene rearrangement studies and isotype analysis suggest that CLL and LCL in RS patients frequently share identical clonal origins. Purine analog therapy of CLL patients does not seem to affect the incidence or clinical behavior of RS. Despite increasing rates of achievement of complete remission in CLL associated with fludarabine-based regimens, RS still occurs, warranting continued surveillance of all CLL patients regardless of disease status. HD and MM in CLL patients are usually advanced at the time of presentation and have poor response and survival rates.