Extracutaneous involvement is a sign of poor prognosis in cutaneous T-cell lymphomas (CTCL). Unfortunately it becomes clinically and histologically manifest only late in the course of the disease. It was the purpose of this study to detect clonality in peripheral blood, lymph nodes and bone marrow samples at times when extracutaneous involvement cannot otherwise be demonstrated. In addition to skin biopsies, peripheral blood, lymph node and bone marrow samples from a total of 25 patients were analysed by Southern blotting for clonal gene rearrangement of the T-cell receptor beta-chain. Six of the patients were suffering from mycosis fungoides (MF), four from non-MF CTCL (pleomorphic T-cell lymphomas), seven from Sézary syndrome (SS), eight from pseudolymphoma (insect bites) (PSL), and one from lymphomatoid papulosis (LP). Clonal TcR b gene rearrangements were found in patients with MF in four of five skin probes as well as in two of two lymph node samples and in one of two peripheral blood samples. In SS patients, all skin probes (seven of seven), lymph node samples (six of six), peripheral blood samples (six of six) and one bone marrow specimen had a clonal TcR beta gene rearrangement. In patients with non-MF CTCL, two of four skin, zero of two peripheral blood and one of one bone marrow samples with clonal T cells were detected. All investigated patients showed exactly the same rearrangement pattern at extranodal sites and in the skin, which is proof for the same clone in all compartments. In contrast, no rearrangements were detected in LP and PSL (zero of eight skin probes, zero of two peripheral blood samples). Our results provide strong evidence for an early systemic spread of neoplastic cells in CTCL. However, an initial tumour burden has to be reached in order to lead to a clinically and prognostically relevant manifestation.