There are no published data on the costs associated with investigating and managing toxicity from ipilimumab treatment in patients with metastatic melanoma. Patients treated with ipilimumab at The Royal Marsden Hospital between 1 September 2010 and 1 April 2013 were identified. Data on demographics, investigations and survival outcomes were collected. Patients with grade 3 or higher immune-related adverse events were identified, and costs of investigating and managing toxicities in them were calculated on the basis of standard National Health Service tariffs. Out of the 110 patients, 29 experienced grade 3/4 immune-related adverse events. The total cost of investigating and managing these patients was £140,680, or a median cost of £2860 per patient. Patients experiencing grade 3/4 toxicities had 1-, 2- and 3-year survival rates of 79, 59 and 46%, compared with 24, 17 and 15% in the group that did not experience significant toxicity (P<0.0005). The most common treatment-related toxicity identified was colitis. Two patients died from complications associated with ipilimumab colitis. The cost of ipilimumab toxicity is marginal in comparison with the total treatment cost. Patients treated with ipilimumab who develop significant toxicity have a higher than expected 1-, 2- and 3-year survival.