B-cell chronic lymphocytic leukemia (B-CLL) is a relatively indolent hematologic malignant disease that, despite a good response to nonaggressive orally administered chemotherapy, currently remains incurable. The overall median duration of survival is more than 5 years, and the presence of anemia or thrombocytopenia adversely affects prognosis. B-CLL is readily diagnosed because of the characteristic and specific phenotypic expressions of the neoplastic cells. Clinical staging continues to be the best prognostic indicator in B-CLL. In addition, cytogenetic status, pattern of leukemic infiltration in the bone marrow, and lymphocyte doubling time are now considered to have additional prognostic value. In this article, the diagnostic evaluation of lymphocytosis is discussed, and an updated analysis on the prognostic determinants of B-CLL is provided. The second part of this clinical update, which reviews current chemotherapeutic modalities, will be published in the subsequent issue of this journal.