Plasma fibronectin concentrations were measured by an ELISA method in two groups of patients with Crohn's disease. In the first study the value of fibronectin as acute phase protein was determined in 48 patients. There was a poor but significant correlation (r = 0.294, p = 0.043) between disease activity and fibronectin concentration in these patients. However, fibronectin was found to show the best correlation with the disease activity of eight acute phase parameters tested. In the second study, the relation between development or presence of fibrosis/stenosis in the intestine of 91 patients with Crohn's disease was related to the plasma fibronectin concentration. The incidence of this type of lesion was found to increase with the level of fibronectin in the plasma together with the disease history of these patients, in particular when intestinal resection had to be performed. These observations reveal that assessment of fibronectin in plasma of patients with Crohn's disease is of limited value for the determination of disease activity, but that its concentration is related to the biological behavior, i.e. formation of fibrosis/stenosis, of the inflammatory lesion.