Acute leukaemia is a rare event during the course of chronic lymphocytic leukaemia (CLL), and only a small fraction of such cases have been shown to be true acute lymphoblastic crises. 1 case is described where both small lymphocytes and proliferating lymphoblasts have the same monoclonal pattern as defined by direct immunofluorescence of membrane‐bound immunoglobulins. Previous cases are reviewed and do not appear to be mere coincidence: acute blast crisis may represent a part of the natural history of CLL.