A drug which makes a wide range of cancers more vulnerable to the body's immune system is "exciting" and may mark a new era, say doctors.
It strips cancer cells of the "camouflage" they use to evade attack by the immune system.
In the most detailed study, published in Nature, some patients completely recovered from terminal bladder cancer.
Cancer Research UK said the field of immunotherapy was delivering "a lot of very exciting results".
The immune system is in delicate balance with some chemicals in the body encouraging a strong vigorous response, while others try to dampen it down.
Tumours can hijack this system to hide from the immune system.
One trick which tumours use is a protein called PD-L1 which is normally used to prevent autoimmune diseases.Bladder
An international team of scientists has been trialling a drug to block PD-L1, produced by the company Roche, on 68 people with advanced bladder cancer.
All the patients had tried chemotherapy and had been given six-to-eight months to live.
More than half the patients, whose tumours were using PD-L1 to hide from the immune system, showed signs of recovery.
In two patients there were no signs of cancer after the treatment.
One in ten patients responded to the experimental therapy even if PD-L1 was not present in the tumour.
Dr Tom Powles, an oncologist at the Barts Cancer Institute and part of the research team, said "There have been no new drugs for bladder cancer for 30 years.
"The tumours have developed a camouflage layer, PD-L1, and by removing the camouflage the tumour becomes identifiable.
"A subgroup of patients seems to do exceptionally well."
Dr Powles is funded by the NHS and receives no money from Roche.
The drug has been given "breakthrough therapy" status in the US and could be used widely by patients there at the end of 2015, if a larger trial shows the same results.
Much larger randomised clinical trials would be needed in order for the experimental therapy to be used in Europe.More cancers
Dr Roy Herbst, who led the research at the Yale Cancer Centre, told the BBC: "This is a new paradigm in cancer.
"Immunotherapy for cancer has opened up an entirely new modality for treatment of the disease alongside chemotherapy, radiotherapy and surgery.
"Now we have immunotherapy and it is here to stay and the challenge for clinicians is seeing how it fits alongside those others."
His study was focused on the environment around a tumour and immune cells there, in order to help predict which patients would respond to therapy.
A similar set of trials to boost the immune attack revealed at the American Society of Clinical Oncology conference in Chicago in June, showed similar therapies could improve survival in advanced skin cancer.
In a trial of 411 patients evaluating a drug, pembrolizumab - 69% of patients survived at least a year.
Those results were described as having the "potential to be a paradigm shift for cancer therapy".
A separate study of 175 patients, led by Yale University in the US, showed responses to the drug in patients with non-small cell lung cancer, melanoma, renal cell carcinoma and other cancers.
Prof Peter Johnson, the chief clinician at Cancer Research UK, said: "We're seeing a lot of very exciting results from these new treatments using the immune system.
"This study in bladder cancer is further proof of the power of this approach, and it's really good to find a new treatment for a type of cancer that we've been struggling to make progress with for many years."