New research indicates that natural killer (NK) cell based adoptive immunotherapy might be an effective treatment of high-grade non-muscle-invasive bladder cancer (NMIBC). Cytokine-activated NK cells derived from patients with NMIBC and from volunteers without cancer were activated in vitro, and the effects of these cells were investigated in stem-like cancer cells and mouse models of NMIBC. Activated NK cells from donors without cancer, but not from those with NMIBC, were able to shift the cells towards a more differentiated, cisplatin-sensitive phenotype. Furthermore, transurethral infusion of NK cells from donors without cancer into mouse models of NMIBC resulted in dramatic reductions in tumour burden.