Antitumour T cells are suppressed by hypoxic tumour areas enriched in extracellular adenosine through activation of A2A adenosine receptors (A2ARs). Hatfield et al. found that mice with lung tumours given supplementary oxygen have improved tumour oxygenation. This enhanced intratumoural infiltration of T cells, reduced immunosuppression by regulatory T cells, and increased tumour regression and long-term survival. As respiratory hyperoxia is used in the clinic, the authors postulate that it can be easily combined with existing immunotherapies for cancer and with available synthetic antagonists of A2ARs.