The outcomes of a phase II, single-arm study involving patients with microsatellite-unstable (MSI) metastatic colorectal cancer receiving treatment with nivolumab reveal high levels of effectiveness. In a heavily pretreated population, 23 patients (31% of the cohort) had an objective response, while 51 (69%) had disease control for at least 12 weeks, after a median follow-up duration of 12 months. These data confirm the promise of previous studies involving immune-checkpoint inhibition in patients with MSI-positive solid tumours.