Soluble CD23 (sCD23) was proposed as a marker of disease activity and as an important prognostic parameter in B-cell chronic lymphocytic leukemia (B-CLL). In this study, prognostic significance of sCD23 in B-CLL was examined according to its temporal relationship with the known clinical parameters of the disease, CD38 and ZAP-70. Serum sCD23 levels of 36 B-CLL patients, followed up in our clinic between 1999 and 2005, and 15 healthy subjects were measured with enzyme-linked immunosorbent assay. The mean serum sCD23 level of the B-CLL patients (210.72 +/- 193.67 and 6-600 U/ml) was significantly higher than the control group (18.20 +/- 14.30 and 6-50 U/ml). Seventy-eight percent of the B-CLL patients with lymphocyte doubling time (LDT) 12 months had high sCD23 levels (P = 0.008). Meanwhile, 81% of the patients with diffuse bone marrow infiltration and 33% of patients with nondiffuse infiltration had high levels of serum sCD23 (P = 0.029). A significant difference was found between B-CLL patients with Binet stages A and C (P = 0.009). Peripheral blood flow cytometry of the patients revealed a significant CD38 expression in patients with high serum sCD23 levels (P = 0.002). Similarly, an increased bone marrow zeta-chain associated protein kinase-70 (ZAP-70) expression was seen in patients with high serum sCD23 levels (P = 0.009, correlation co-efficient was 0.714). Cumulative and the progression free survivals of the patients with low serum sCD23 levels were 60.1 +/- 5.7 months [95% confidence interval (CI); 49.0-71.2] and 51.1 +/- 6.6 months (95% CI; 38.0-64.1), respectively. However, they were 43.8 +/- 6.5 months (95% CI; 31.0-56.6) and 26.5 +/- 6.4 months (95% CI; 14.0-39.1) in patients with high levels. Serum sCD23 is increased in B-CLL patients and can be used in the clinical follow-up of the disease in prediction of the tumor mass and prognosis.