The immunophenotype and genotype of atypical cells in skin and lymph node infiltrates were investigated in a patient with lymphomatoid papulosis (LyP) complicated by anaplastic large-cell lymphoma of the lymph nodes. The large atypical cells in both skin and lymph nodes displayed an almost identical immunophenotype, i.e. CD30+ and CD25+. Southern blot analysis for T-cell receptor beta-chain gene rearrangement revealed an identical gene configuration in DNA extracted from skin and lymph node. Our results strongly support the hypothesis that clonal populations of T cells arising in cutaneous LyP lesions may undergo malignant transformation, spread into regional lymph nodes, and give rise to secondary malignant lymphomas, such as anaplastic large-cell lymphoma.