The prognosis of Sézary syndrome (SS) and mycosis fungoides (MF) depends on lymph node (LN) involvement. The usefulness of LN image guided core needle biopsies (CNB), instead of surgical sampling, has been poorly evaluated.To determine the prognostic value of LN CNB in MF/SS.A retrospective search was conducted to identify all LN biopsy specimens of MF/SS between 2008 and 2019. Biopsies were staged according to the ISCL/EORTC criteria. We performed immunolabelings and determined the tumor clone frequency (TCF) by high-throughput sequencing of the T-cell receptor ß gene.We included 119 consecutive biopsies from 100 patients with 45 MF and 55 SS. N1, N2, and N3 stages were diagnosed in 34 (29%), 26 (22%), and 59 (49%) cases. The percentage of TOX, PD1, KIR3DL2, and CD30 positive cells, the Ki67 index and the TCF were all positively correlated to the N stage. The median overall survival (OS) was 42 (26-NR) versus 14 (5-30) months for N1/N2 versus N3 patients (p < 0.001). In univariate analyses, an age>75 years, LN short-axis diameter>15 mm, N3 stage, the presence of large cell transformation, TOX>60%, PD1>25%, Ki67>30%, KIR3DL2>15%, CD30>10%, and TCF>25% were identified as adverse prognostic factors. In multivariate analyses, only an age>75 years and Ki67 index>30% were associated with reduced OS. We developed a new prognostic index associating the N stage and the Ki67 index, which better discriminates N3 patients with poor prognosis.CNB allows an objective assessment of the LN involvement in MF/SS, relevant for staging and prognosis.