The use of cancer immunotherapies can result in severe toxic effects. As cancer is primarily a disease of ageing, Mirsoian et al. have examined the toxicity of immunotherapy in aged mice. The authors previously showed that treatment of young mice with an immune-stimulatory therapy including interleukin-2 (IL-2) and a monoclonal antibody against CD40 (anti-CD40) reduced tumour growth and was well tolerated. However, in aged mice this treatment induced a cytokine storm that was 100% lethal. They have now found that levels of visceral fat in mice increase with age. Young obese mice have similar levels of visceral fat to aged mice, and IL-2 and anti-CD40 treatment also induces a lethal cytokine storm in these mice, whereas calorie-restricted aged mice have less visceral fat and are protected from treatment toxicity.