Summary The differential diagnosis between Kaposi's sarcoma and the so-called ‘pseudo-Kaposi's sarcoma’ or acroangiodermatitis of the feet is often fraught with difficulty, not only on clinical but also on histological grounds. The aim of this study was to assess whether immunolabelling for the CD34 antigen, a marker of Kaposi's sarcoma cells, could be of value in the distinction between these two angioproliferative disorders. We comparatively examined 16 biopsy specimens from cases of Kaposi's sarcoma and seven biopsies from patients with pseudo-Kaposi's sarcoma, by a streptavidin-biotinperoxidase method, using a monoclonal antibody to the CD34 antigen. All cases of Kaposi's sarcoma showed CD34 labelling both on endothelial cells and on the characteristic spindle-shaped, perivascular cells. Biopsies of pseudo-Kaposi's sarcoma showed a strong labelling of endothelial cells of hyperplastic vessels. However, in sharp contrast with Kaposi's sarcoma, a complete absence of perivascular CD34 expression was noted. It seems therefore that immunolabelling for the CD34 antigen appears to be a valuable tool in the differential diagnosis between Kaposi's sarcoma and pseudo-Kaposi's sarcoma.