Increased levels of soluble intercellular adhesion molecule 1 (sICAM-1) in serum have been demonstrated in several human disease conditions. We have previously shown, in a point-prevalence study, a positive correlation between sICAM-1 levels and disease relapse in patients with idiopathic retinal vasculitis. We now report a longitudinal study over 1 year in which sICAM-1 levels were compared with clinical disease status in order to determine this relationship further. Serum samples from 11 patients with idiopathic retinal vasculitis were tested for the presence of sICAM-1 by enzyme linked immunosorbent assay. Eight control subjects were also tested. Five out of 11 patients presented with relapse and had raised sICAM-1 levels compared with quiescent periods of their disease. Five out of 11 patients showed no relapse over 1 year and also no increase in sICAM-1 levels. One patient showed increased levels of sICAM-1, but no clinical signs of relapse. These results indicate that sICAM-1 is associated with disease activity in retinal vasculitis patients and could indicate dysfunction of the blood-retina barrier.