Sarcoidosis is characterized by a cell-mediated response mediated by the activation of CD4+ T lymphocytes in an environment lacking adequate numbers of regulatory CD8+ T lymphocytes. Immunohistological studies on frozen tissues have shown that sarcoid lesions have activated CD4 helper/inducer T lymphocytes at the centre of granulomata, whereas lymphocytes at the periphery are mainly CD8 suppressor/cytotoxic cells. In this study we investigated the immunohistological distribution of CD45 isoforms of T cells in 29 paraffin-embedded sarcoid lesions in mediastinal and open lung biopsies. Ten of these were assessed quantitatively, with single-staining of serial sections demonstrating a predominance of CD45RO memory T lymphocytes in granulomata and intergranulomatous areas. Ratios of CD45RO:CD45RA T lymphocytes (or the ratio of memory to naive T cells) were 42.0:1 for granulomata and 17.9:1 for intergranulomatous areas of sarcoid lesions counted. This finding is compatible with the hypothesis that nearly all the lymphocytes present in sarcoid lesions have been previously activated, and selectively home to sarcoid lesions.