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The evaluation of chronic pain is challenging because of the lack of specific biomarkers. We identified the Mu opioid receptor-positive (Mu+) B cell percentage of expression, named Mu-Lympho-Marker (MLM), as a candidate marker for chronic pain in fibromyalgia (FM) and osteoarthritis (OA) patients. Here, we investigate the role of MLM on natural killer (NK) cells in the same patients. Twenty-nine FM and twelve OA patients were analyzed, and twenty-three pain-free subjects were considered as the control group. Blood samples were collected to perform immunophenotyping and Western blot analysis. Biological and clinical data were statistically analyzed. The final results showed that the percentage of NK cells expressing Mu was statistically lower in FM and OA patients than in pain-free subjects, as already demonstrated for B cells. A Western blot analysis was performed in order to detect NK cells’ functional status. Moreover, the correlation analysis of MLM expression with pharmacological therapy did not show any significant results. In conclusion, here, we confirm the role of MLM as a suitable marker for chronic pain and underline NK cells as a new possible immune cell type involved in the “Mu opioid receptor reserve theory”.