The treatment of any malignant disease requires appropriate criteria for diagnosis of the disease, appropriate methods for study, and techniques for evaluating response to treatment. In the past five years, there have been a number of proposed guidelines for studies in chronic lymphocytic leukemia (CLL). These submitted guidelines represent a further re-evaluation of concepts in CLL, and are currently in use by Cancer and Leukemia Group B for protocol studies. It is clear that revisions will continue to be make as new knowledge accumulates. In the meantime, these are submitted for publication for use by other investigators.