Intercellular adhesion molecule (ICAM)-1 promotes the initial interaction between macrophages and T cells during immune activation. We have measured serum levels of soluble ICAM-1 (sICAM-1) by ELISA in 27 patients with ulcerative colitis (UC), 31 with Crohn's disease (CD), and 29 healthy subjects. The median sICAM-1 serum concentration was significantly increased in inflammatory bowel disease (IBD) patients (355 ng/ml, range 195-855) compared to controls (245 ng/ml, 155-580) (P = 0.001). Variance analysis for trend showed that sICAM-1 levels were significantly higher in patients with active CD and UC, compared to those with inactive disease and controls (P = 0.00002). The concentration of sICAM-1 was higher in CD patients (365 ng/ml 230-470) compared to UC (300 ng/ml 195-855) (P = 0.01). Furthermore, weak but significant correlations were found between serum levels of sICAM-1 and: soluble IL-2 receptors, orosomucoid, and C-reactive protein. It is suggested that increased circulating sICAM-1 levels may reflect increased adhesiveness and signal transmission across cells, probably as a result of shedding of the parent molecule during local cellular immunoresponses in vivo.