We describe a patient with B cell chronic lymphocytic leukemia whose B lymphocytes showed the capacity to form rosettes with sheep red blood cells during prolymphocytoid transformation of the disease. In this case, rosette formation was found to be related to the presence of the classic E rosette receptor since the leukemic cells were recognized by the OKT11 monoclonal antibody. This feature was not shown at diagnosis. Evaluation of this phenomenon is discussed.