What is already known about this subject?There is evidence that NK cell functionality is severely altered in obesity. Our group and others have shown distinct loss of function (e.g. cytotoxicity and cytokine production) of NK cells from individuals with obesity and experimental animals. What does this study add?For the first time, the present study demonstrates a re-activation of NK cell functionality after fat mass reduction as a consequence of a combined exercise and dietary program in person with obesity. Conclusion:The present study demonstrates a re-activation of NK cell functionality after body fat mass reduction in person with obesity.