Novel cancer immunotherapy antibodies are moving from clinical trials into routine practice, delivering sustained benefits and prolonged survival to patients with melanoma, lung, kidney and other cancers. These immunostimulatory antibodies non-specifically activate the patient's own immune system by inhibiting immune system checkpoint proteins. This mechanism of action is entirely different to traditional cancer treatments, such as chemotherapy. While there are virtually no immediate toxicities, serious life-threatening autoimmune side-effects such as colitis, dermatitis, hypophysitis, pneumonitis and hepatitis can occur, sometimes starting long after the treatment has been given. Recognition, referral and prompt treatment with immunosuppressive drugs like corticosteroids can control these immune-related side-effects without compromising efficacy. This exciting new class of drugs is defining a new paradigm in cancer therapy.