Myalgic Encephalomyelitis (ME) is a severe and multisystemic illness with symptoms including post-exertional neuroimmune exhaustion, gastrointestinal complaints, neurocognitive disturbances, orthostatic intolerance, and impaired ion transport. There is no laboratory diagnostic test nor evidence-based treatment for ME. A major laboratory finding was the discovery of significantly reduced natural killer (NK) cell cytotoxicity in ME. Research was undertaken across a series of studies involving n=607 people with ME and n=489 non-fatigued controls (NFC) using flow cytometry, confocal microscopy, and genome studies. Research was conducted using peripheral blood mononuclear cells (PBMCs) or isolated NK cells to investigate NK cell phenotypes, surface receptors, cytokine production, degranulation, lytic proteins, protein phosphorylation, and ion transport. There was a significant increase in CD56^Bright NK cells in ME compared with NFC, an increase in CD56^Dim NK cells was associated with moderate-severe ME (p<0.05). A significant reduction in NK cell cytotoxicity was consistently observed in ME compared with NFCs (p<0.05). There was a significant increase in degranulation and a significant reduction in perforin and granzyme B (p<0.05). A significant increase in interferon-gamma (IFN-γ) in ME compared with NFCs (p<0.05). There was a significant reduction in mRNA coding for lytic proteins and IFN-γ (p<0.05). A significant reduction in phosphorylation of extracellular signal-related kinases and an increase in mitogen activated kinases and p38 was observed in ME (p<0.05). Impaired calcium mobilisation was reported in ME compared with NFCs (p<0.05). Single nucleotide polymorphisms were identified in calcium-permeable transient receptor potential melastatin (TRPM) 3 ion channels (p<0.05). Surface expression of TRPM3 was significantly reduced on NK cells of ME compared with NFCs (p<0.05).  Impaired TRPM3 was consistently reported (p<0.05). The culmination of this research demonstrates that NK cells provide a reliable cell model for research into the pathophysiology of ME and TRPM3 ion channel activity provides an opportunity for diagnosis and pharmacotherapeutics.