Different lines of evidence support a major role for activated T-lymphocytes in the pathogenesis of posterior uveitis. The initial site of activation of these autoreactive T-cells, either locally in the eye or in the peripheral immune compartment, is still unknown. This study was undertaken to investigate whether with currently available techniques, it is possible to detect alterations in the levels and subsets of activated T-cells in the peripheral blood of patients with posterior uveitis.For this reason, 3-colour immunofluorescent staining was performed to assess the distribution of IL-2 receptors (IL-2R) and the CD45RO-antigen on CD4+ and CD8+ subsets of peripheral blood lymphocytes (PBLs) from patients with posterior uveitis (n = 29).Only the subgroup of patients with posterior uveitis as part of a systemic immune-mediated disease (sarcoidosis, Behcet's disease) (n = 9) showed a significant increase in IL-2R expression on peripheral blood lymphocytes (p<0.005) when compared to normals (n = 12). This increased expression was reflected much more significantly in the CD4 + (p<0.0005) rather than in the CD8 + subset (p<0.05) of lymphocytes.In contrast, no significant increase in CD45RO expression on either subset of T lymphocytes was found in any subgroup of posterior uveitis in comparison with normals.