The authors report 6 cases of histological transformation in peripheral T-cell lymphomas of low grade of malignancy. The transformation occurs in 75% of the cases, in extra-nodal sites and corresponds to a monomorphic of pleiomorphic large cell type. There is no discordance in the immunologic results between the two phases. The transformation seems to occur earlier than in B lymphomas, with a relative frequency of hematological manifestations and no therapeutic response.