Extranodal natural killer/T-cell lymphomas (NK/TL), rare in Europe, are Epstein-Barr virus (EBV) associated lymphomas with poor outcomes. Here, we determined the virus type and analyzed the EBV latent membrane protein-1 (LMP1) sequence in NK/TL from French patients. Six clones of viral LMP1 were sequenced by Sanger technology in blood from 13 patients before treatment with an L-asparaginase based regimen and, for 8 of them, throughout the treatment. Blood LMP1 sequences from 21 patients without any known malignancy were tested as controls. EBV type A was identified for 11/13 patients and for all controls. Before treatment, a clonal LMP1 gene containing a 30bp deletion (del30) was found in 46.1% of NK/TL and only in 4.8% of controls. Treatment was less effective in these patients who died more rapidly than the others. Patients with a deleted strain evolving towards a wild type strain during treatment reached complete remission. The LMP1 gene was sequenced by highly sensitive next-generation sequencing technology (NGS) in 5 NK/TL nasopharyngeal biopsies, 2 of them originating from the previous patients. Del30 was present in 100% of the biopsies; 2 viruses at least coexisted in 3 biopsies. These results suggest that del30 may be associated with poor prognosis NK/TL and that strain evolution could be used as a potential marker to monitor treatment. This article is protected by copyright. All rights reserved.