The autoimmune lymphoproliferative syndrome (ALPS) is characterized by non-malignant lymphoproliferation and signs of autoimmunity. A hallmark of ALPS are high amounts of circulating CD3+/CD4-/CD8- double negative T-lymphocytes (DN T cells). The origin of these cells remains elusive. To investigate the relationships of DN T cells and the single positive T cell populations (CD4+ and CD8+), we analyzed by spectratyping the complementarity determining regions 3 (CDR3) of the T cell receptors in sorted "single positive" (CD4+, CD8+) and DN T cells in a patient with ALPS type 1a. We observed signs for clonal expansion in all three T cell subpopulations. Strong and weak clonal expansions were to be seen in 16 and 14 for DN, 6 and 12 for CD8+, and 1 and 5 for CD4 + T cells, respectively. Most importantly, 24 out of 30 aberrant peaks in the spectratype histograms of the DN T cells where unique for this population and were not to be detected in the histograms of the single positive T cells. In contrast to published data, we conclude that expanded DN T cell populations in ALPS are not generally derived from expanded CD3+/CD4+ or CD3+/CD8+ populations.