There is accumulating evidence that exercise can suppress tumorigenesis in humans and improve outcomes in patients with cancer but the mechanisms are unclear. Using five mouse models of cancer, Pedersen et al. showed that voluntary running reduced tumorigenesis and also reduced tumour progression by ∼60%. These reductions were associated with increased interleukin-6 (IL-6)-sensitive natural killer (NK) cell infiltration of tumours and these cells were mobilized by running-induced epinephrine, This study raises many questions. In particular, as mice are naturally very active it is interesting to debate whether this demonstrates that increased exercise suppresses tumorigenesis and tumour progression or that increased sedentary behaviour promotes them.