The pathogenesis of lymphoedema in patients infected with Wuchereria bancrofti or Brugia malayi remains unclear. Lymph stasis and local immunological reactions seem to play the main role. In order to discriminate between the obstructive and immunological effects of the parasite, a comparative histological study of skin specimens obtained from two groups of patients, one with filarial and the other with postsurgical lymphedema of lower extremities, was performed. In both groups patients suffered lymph stasis, in the first due to filariasis, in the other due to removal or irradiation of pelvic lymph nodes. The patients with filarial infection showed hyperproliferation of keratinocytes, focal acantholysis, accumulation of lymphocytes at the epidermo-dermal junction, profuse pericapillary and perivenular mononuclear infiltrations in the dermis marginated granulocytes in capillaries and, in some cases, subepidermal granulocytic infiltrates. There were many dilated initial lymphatics and lymphatic "lakes" between thick collagen fibre bundles. Monoclonal antibody analysis revealed that the most common cells in the infiltrates were macrophages (CD68+). All mononuclear and endothelial cells were HLA-DR+. In contrast, the skin specimens of non-filarial patients revealed only moderate proliferation of keratinocytes, increased numbers of CD1+ epidermal Langerhans cells, moderate pericapillary infiltrates of CD68+, CD4+ and CD8+ cells, and evidently less intensive marking of cells with anti-HLA-DR antibody. There were few initial lymphatics visible. These findings indicate that filarial lymphoedema is complicated by a severe inflammatory component, which is much less expressed in postsurgical lymph stasis.