Distinguishing chronic lymphoproliferative disorders of NK cells (CLPD-NK) from reactive NK cell expansions is challenging. We assessed the value of NK receptor phenotyping and targeted high-throughput sequencing in a cohort of 114 consecutive patients with NK cell proliferation, retrospectively assigned to a CLPD-NK group (N=46) and a reactive NK group (N=68). We then developed a NK-clonality score combining flow cytometry and molecular profiling with a positive predictive value of 93%. STAT3 and TET2 mutations were respectively identified in 27% and 34% of the CLPD-NK patients - constituting a new diagnostic hallmark for this disease. TET2-mutated CLPD-NK exhibited preferentially a CD16low phenotype, displayed more frequently a lower platelet count, and were associated with other hematologic malignancies such as myelodysplasia. To explore the mutational clonal hierarchy of CLPD-NK, we performed a whole exome sequencing of sorted, myeloid, T, and NK cells and identified that TET2 mutations were shared by myeloid and NK cells in 3 out of 4 cases. Thus, we hypothesized that TET2 alterations occur early in CLPD-NK disease which could explain a potential link between NK-LGL leukemia and other myeloid malignancies. Finally, we analyzed the transcriptome by RNA-seq of 7 CLPD-NK and evidenced two groups of patients. The first group displayed STAT3 mutations or SOCS3 methylation and overexpressed STAT3 target genes. The second group, including two TET2-mutated cases, significantly under-expressed genes known to be down-regulated in angioimmunoblastic T-cell lymphoma. Our results provide new insights into the pathogenesis of NK cell proliferative disorders and potentially new therapeutic opportunities.