In this multicenter study that collected data from 22 Thai children and adolescents diagnosed with SPTCL over the past 16 years, we discovered that most of the cases harbored germline HAVCR2 mutations (86.4%), either homozygous or heterozygous, and manifested with HLH/HLH-like systemic illnesses (68.1%) at the time of SPTCL diagnosis. The gene discussed is HAVCR2; the disease is subcutaneous panniculitis-like T-cell lymphoma.