Gene rearrangement analysis has been performed in 23 patients with T-cell lymphoproliferative diseases: 4 cases with T-gamma lymphocytosis, one case of a Sezary's syndrome, one case of T-cell angioimmunoblastic lymphoma, two cases of T-cell lymphoepitheloid lymphoma, 11 patients with T-cell pleomorphic lymphoma, 3 cases of large anaplastic T-cell lymphoma and one case of T-cell lymphoblastic lymphoma. Rearranged banding patterns have been observed for at least one of the T-cell receptors (TCR) in 19 of the cases, and germ line configuration of the TCR and Ig genes in the other four. Likewise, both Ig and TCR rearrangements have been observed in three cases (one case of T-cell pleomorphic lymphoma, one case of large anaplastic T-cell lymphoma and one case of T-cell lymphoblastic lymphoma). Molecular genetic techniques have been used in order to direct monoclonal proliferations of T cell in the tumoral tissues, to determine the T- or B-cell lineage of the neoplasia and also to outline the molecular characteristics of each group that constitutes the complex classification of T-cell malignancies.