CD56brightCD16dim NK cells expressing KIR3DL1/DL2 and CD56brightCD16+ NK cells expressing KIR2DS4 and KIR2DL2/DL3 were significantly increased after 6 months in controls and moderate CFS/ME patients, while severe CFS/ME patients showed significantly increased CD56brightCD16+ NK cells expressing the KIR2DL1 receptor after 6 months. Here, KIR2DS4 is linked to myalgic encephalomeyelitis/chronic fatigue syndrome.