This study was undertaken to determine the clonality of lymphomatoid papulosis (LyP), its clonal relationship to lymphomas, which occur at high frequency in LyP patients, and to define the cell lineage of Reed-Sternberg-like cells in type A lesions of LyP. Punch biopsies of skin of 11 adult patients with LyP were analyzed for morphologic subtype of LyP, surface antigens, and clonal T-cell receptor (TCR) gene rearrangements. Clonal rearrangements were identified by semiquantitative polymerase chain reaction amplification and sequencing of TCR-beta chain genes in nine patients and TCR-gamma chain genes in two patients. A single dominant clone was detected in multiple separate LyP lesions, often of different histologies, in nine patients. The same clone was detected in LyP lesions and the anaplastic large cell lymphoma (ALCL) of 2 patients and the mycosis fungoides (MF) of 2 other patients. No dominant clone could be detected in one patient with LyP uncomplicated by lymphoma or in a second patient with LyP and MF. A T-cell lineage was evident for RS-like cells in cell culture and in type A lesions. These results show that multiple regressing skin lesions and associated T cell lymphomas (MF and ALCL) are clonally related in most LyP patients, which suggest that the disease in these patients was initiated by a non-random genetic event.