In the present study, STAT3 gene mutations were evaluated using next-generation sequencing (NGS) in a cohort of 25 patients with autoimmune rheumatic diseases (ARDs), clinical presentation typical for T-LGL leukemia, clonal rearrangement of the T-cell receptor (TCR) gene, and absolute circulating LGL count of less than 0.5 × 109/L. The gene discussed is STAT3; the disease is T-cell large granular lymphocyte leukemia.