BACKGROUND: Detection of clonality has been reported to be a helpful tool in the diagnosis of cutaneous lymphomas. Monoclonal rearrangement of T-cell receptor genes (TCR) was reported in fresh frozen tissue of lymphomatoid papulosis (LyP) and primary cutaneous anaplastic large-cell lymphoma (ALCL), but the diagnostic value of T-cell clonality in formalin-fixed, paraffin-embedded biopsies has so far not been assessed. METHODS: Detection of clonal rearrangement of TCRgamma genes by highly sensitive polymerase chain reaction-based automated high-resolution fragment analysis (AHRFA) in archival LyP (n = 18) and ALCL (n = 17) tissue. RESULTS: Detection of clonality differed significantly among the histologic forms of LyP as well as between LyP and ALCL with clonality found in none of the 10 biopsies of LyP type A and B, in 4/8 (50%) of the LyP type C specimens, and in 11/17 (65%) of ALCL cases. CONCLUSIONS: T-cell clonality can only be found in a minority (four of 18; 22%) of archival LyP specimens, even when employing a highly sensitive detection method and is thus of limited diagnostic value. Final diagnosis of LyP has to be based mainly on clinical, histologic, and immunohistochemical findings rather than on results of clonality studies.