Mitochondrial function plays a central role in regulating immunological and metabolic processes, particularly during successful aging. This cross-sectional study aimed to investigate associations between mitochondrial respiration of peripheral blood mononuclear cells (PBMCs; MR(PBMC)) and key markers of immune function, systemic inflammation, and metabolic health in a cohort of healthy older adults. Sixteen healthy, physically active participants aged > 55 years (male: n = 9; female: n = 7; age: 64 ± 3.7 years; BMI: 24.3 ± 2.9; VO(2peak): 31.1 ± 8.8 mL/min/kg) were recruited. Participants were tested for their maximal oxygen uptake (VO(2peak)) as well as cardiovascular and metabolic risk factors. Venous fasting blood samples were collected. For further analysis, MR(PBMC) was measured using the Oroboros O2k-Oxygraph. T cell subsets were analyzed by flow cytometry, serum cytokines by LUMINEX assays, and gene expression by qPCR analysis. We found positive associations between basal and maximal MR(PBMC,) and the percentage of CD4(+) T cells, with a notable link to naïve CD4(+) T cells (p < 0.05). Maximal MR(PBMC) was negatively associated with proportion of effector memory CD4(+) T cells (p < 0.05). Basal MR(PBMC) showed negative associations with pro-inflammatory serum cytokine tumor necrosis factor alpha (TNF-
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