B-Chronic lymphocytic leukaemia (B-CLL) is a monoclonal malignancy characterized by an accumulation of terminally differentiated small and anergic B lymphocytes in the blood, bone marrow and other tissues. CD23 antigen, a trans-membrane glycoprotein, promotes the activation and proliferation of normal B lymphocytes and has an important role in the process of malignant transformation in B-CLL. This retrospective cohort study of 77 consecutive newly diagnosed B-CLL patients, 43 males, 34 females, median age of 62 years, examined CD23 expression and correlations with clinical parameters. CD23+ was negatively correlated with pro-lymphocyte infiltration of the bone marrow (P<0.01) and peripheral blood lymphocyte counts (P<0.001). Lower CD23 expression was correlated with lower serum immunoglobulin levels (P<0.05), especially IgG; while greater CD23 expression was positively correlated with higher CD5 levels. B-CLL patients with a percentage of CD23+ lymphocytes >40% had longer survival (92.8 months) than those expressing <40% (35.3 months) (P=0.001). CD23 is not uniformly expressed by lymphocytes in B-CLL patients, and the differences in expression are dependent on a number of clinical parameters, including the peripheral blood lymphocyte count and the degree of pro-lymphocyte infiltration of the bone marrow. CD23 expression is significantly decreased in patients with extremely high lymphocyte counts (PBL counts of >100 x 10(9)/l) and in the advanced stages of disease.