Genotypic and immunological analysis was performed in 10 patients with anaplastic large cell lymphoma (Ki-1 lymphoma), 4 were male and 6 female. Immunophenotypically, 9 of these patients expressed some T cell markers; CD 2 in 9, CD 4 in 9, CD 3 in 6, CD 8 in 4, and UCHL-1 in 8. For B cell markers, 4 patients expressed B 1 and one expressed L 26; these patients also expressed T cell markers. In genotypic analysis, 9 of 10 cases displayed some incidence of T cell receptor gene (TCR) rearrangement or deletion, which was found in 3 patients for C beta 1, 7 for C beta 2, 8 for J gamma, 5 for C gamma (Hind III), 5 for C gamma (EcoRI), 3 for J delta 1 (Hind III), 3 for J delta 1 (BamHI), 3 for J delta 2, and 2 for C delta. One patient with rearrangement of TCR also showed rearrangement of immunoglobulin heavy chain. Another patient exhibited rearrangement of both TCR and immunoglobulin chain. These results indicate a common T cell lineage for this type of lymphoma.