Lymphomatoid papulosis (LyP) is a rare cutaneous lymphoproliferative disorder in children, which can rarely be associated with a cutaneous or systemic lymphoma. We report a 13-year-old girl who presented with typical LyP and pathological features of subtype A. Six months later, the patient presented with rapidly progressive peripheral and systemic lymphadenopathy. On examination of a lymph-node biopsy, a lymphoid infiltrate negative for anaplastic lymphoma kinase (ALK) and positive for CD30 was found, suggestive of systemic anaplastic large T-cell lymphoma (S-ALCL). The patient was treated with chemotherapy, followed by allogeneic bone-marrow transplant (BMT). Over the following 6 years, she presented with biopsy-confirmed LyP relapses with complete cutaneous, peripheral-blood and bone-marrow chimerism. This is only the third reported paediatric association of S-ALCL with LyP to our knowledge, and seems to be the first paediatric case of recurrent relapses of LyP after bone-marrow allograft for S-ALCL with total (100%) cutaneous and bone-marrow chimerism. LyP occurring after allogenic BMT does not appear to be donor-derived.