The serum levels of soluble intercellular adhesion molecule 1 (ICAM-1) were significantly elevated (P < .001) in patients with chronic B- lymphocytic leukemia (B-CLL, n = 113) compared with healthy controls (n = 31). sICAM-1 levels in B-CLL were positively correlated to the tumor mass as reflected by the modified Rai and the Binet staging systems, lymphocyte counts, and isolated spleno/hepatomegaly. During disease progression or regression on cytoreductive therapy, the circulating sICAM-1 levels changed accordingly. sICAM-1 was also correlated to a kinetic parameter such as the lymphocyte doubling time. Furthermore, the serum sICAM-1 levels were inversely correlated to hemoglobin levels in patients with early clinical stage, and this may turn out to be of prognostic value. sICAM-1 was compared with other serum markers said to reflect disease activity in B-CLL, ie, soluble CD23, thymidine kinase, lactate dehydrogenase (LDH), and beta 2-microglobulin. sICAM-1 was equally well or better correlated to clinical stage and lymphocyte doubling time. In univariate regression analysis, all serum markers but LDH correlated with survival, and in multivariate analysis, sICAM-1 was the only marker approaching significance for additional prognostic information when included after clinical stage and lymphocyte doubling time. Based on the present observations, it appears that prospective studies repeatedly monitoring serum sICAM-1 in B-CLL are justified.