Epstein-Barr virus (EBV) associated T-cell and NK-cell lymphoproliferative diseases are lethal and extremely rare in Caucasians. We expand on the clinical, immunological and histogenetic characteristics associated with this second European case (19 years old, previously healthy, Caucasian boy) of systemic EBV positive T-cell lymphoma of childhood. We report, as novel findings, severe lympho-depletion and abrogation of thymopoiesis secondary to severe EBV activation and excessive immune activation. Similar to the first European case, we also detected a somatic missense variant in the proto-oncogene FYN. In the first European patient however, the FYN variant allele frequency (VAF) was 10% and the patient only experienced moderate leukopenia, whereas in our case, the VAF was 48% and the patient experienced severe leukopenia and lymphopenia. This could suggest a pathogenic role of these FYN variants in driving excessive T cell activation. If confirmed, FYN might become target in future treatments of this fatal disorder.