A soluble form of intercellular adhesion molecule-1 (sICAM-1) has been described in serum and other body fluids. In order to determine whether sICAM-1 in serum and urine is a useful marker of inflammatory activity in kidney diseases we measured sICAM-1 in serum and urine of fifty patients who underwent renal biopsy, and of twenty healthy individuals. Expression of ICAM-1 on proximal tubular epithelial cells was investigated by immunohistochemistry. Soluble ICAM-1 in serum did not differ between patients and controls (354 +/- 129 ng/ml vs. 305 +/- 52 ng/ml). By multiple regression analysis sICAM-1 correlated with tubular expression of ICAM-1 (p < 0.01), but not with serum creatinine, infiltrating leukocytes, urinary ICAM-1 or proteinuria. In healthy controls mean urinary ICAM-1/cr was 2.5 +/- 3.0 ng/mg creatinine and differed significantly from that of patients (14.5 +/- 14.9 ng/mg) (p < 0.005). Patients with minimal-change disease had the highest uICAM-1 levels. The ratio of urinary ICAM-1 and proteinuria was remarkably constant in all patients with 6.0 +/- 0.9 ng/mg. By multiple regression analysis uICAM-1/cr correlated with proteinuria/cr (p < 0.001) and sICAM-1 (p < 0.005). These data show that sICAM-1 does to some degree reflect ICAM-1 expression in the kidney, whereas uICAM-1 is derived from glomerular filtration and closely parallels proteinuria. Both sICAM-1 and uICAM-1 are not useful to estimate ICAM-1 expression and inflammatory activity in the kidney.