This study was initiated to evaluate whether soluble CD23 (sCD23) reflects disease activity and tumor load in B-cell chronic lymphocytic leukemia (B-CLL) and to determine its prognostic potential for this disease. The concentration of sCD23 was measured in the serum of 45 B-CLL patients, 50 patients with other lymphoproliferative disorders, and 41 healthy donors (HD). sCD23 serum levels from B-CLL patients were correlated with parameters of disease activity and total tumor mass (TTM) score. In selected cases, sCD23 was measured repeatedly over a 24-month period. Expression, density, and calculated total amount of membrane CD23 on peripheral-blood B-CLL cells, as well as its correlation to sCD23 levels in serum and supernatants, were determined. sCD23 in B-CLL patients serum was highly elevated as compared with other lymphoproliferative disorders, with the exception of immunocytoma (IC). Both advanced Rai stages and active forms of B-CLL were associated with higher levels of sCD23. There was a highly significant reciprocal relationship between sCD23 and lymphocyte count doubling time (LCDT). Serum sCD23 correlated positively with serum deoxythymidine kinase activity and TTM score, but not with absolute lymphocyte counts. The repetitive measurement of serum sCD23 showed the usefulness of this marker in monitoring disease progression in B-CLL. The total amount of membrane CD23 on in vitro-cultured B-CLL cells correlated significantly with sCD23 levels in the supernatant, whereas correlation between serum sCD23 and membrane CD23 on freshly isolated B-CLL cells was absent. Our results indicate that sCD23 is a highly sensitive and specific parameter with prognostic potential for B-CLL, which may be used as a tumor marker and may help to assess disease activity.