The serum levels of soluble ICAM-1 (CD54) were significantly elevated in patients with non-Hodgkin's lymphomas (NHL, n=127) and hairy cell leukaemia (HCL, n=15) compared with healthy controls (n=31). In high-grade malignant NHL (n=79) the sICAM-1 levels correlated with the tumour mass as reflected in the Ann Arbor staging system but not with bulky disease. Further, the sICAM-1 levels correlated with disease activity as reflected by the presence of B symptoms and with other known prognostic markers. In particular serum thymidine kinase (sTK). In patients with low-grade malignant NHL (n=48) a trend towards higher serum levels of sICAM-1 was found in patients with advanced stage and B symptoms. In both low and high-grade malignant NHL, elevated levels of sICAM-1 were associated with poorer overall and disease-free survival. The present results indicated that sICAM-1 levels have a prognostic power equal to that of other serum markers claimed to be of prognostic value in NHL, namely serum lactate dehydrogenase (LDH), erythrocyte sedimentation rate (ESR), beta-2-microglobulin (beta2m), serum thymidine kinase (sTK), albumin and orosomucoid. The cellular origin and the possible interactions between soluble and surface ICAM-1 and its ligands needs further exploration.