A genetically engineered version of a virus that normally causes cold sores shows real promise for treating skin cancer, say researchers.
The modified herpes virus is harmless to normal cells but when injected into tumours it replicates and releases substances to help fight the cancer.
Trial results published in the Journal of Clinical Oncology show the therapy could lengthen survival by years - but only for some melanoma patients.
The treatment is not yet licensed.
Similar "immunotherapy" treatments for melanoma are already available in the US and in Europe, but researchers believe T-Vec would be a welcome addition to these.
It would also be the first melanoma treatment that uses a virus.
The latest study is the largest ever randomised trial of an anti-cancer virus and involved 436 patients from 64 centres in the US, the UK, Canada and South Africa who had inoperable malignant melanoma.
UK trial leader Prof Kevin Harrington, from the Institute of Cancer Research, London, said: "There is increasing excitement over the use of viral treatments like T-Vec for cancer, because they can launch a two-pronged attack on tumours - both killing cancer cells directly and marshalling the immune system against them.
"And because viral treatment can target cancer cells specifically, it tends to have fewer side effects than traditional chemotherapy or some of the other new immunotherapies."
Dr Hayley Frend, science information manager at Cancer Research UK, said: "Previous studies have shown T-Vec could benefit some people with advanced skin cancer, but this is the first study to prove an increase in survival.
"The next step will be to understand why only some patients respond to T-Vec, in order to help better identify which patients might benefit from it."
Although it has not yet been licensed, doctors are excited about the very real prospect of a brand new type of treatment for advanced melanoma - and, in the future, possibly other cancers too.
The idea of using viruses to enter and kill cancerous cells has been gathering scientific pace and kudos.
This latest study in the Journal of Clinical Oncology is the largest ever randomised trial of an anti-cancer virus and provides tantalising evidence that the treatment concept could soon be moved into the clinic, after decades of work in the lab.
Researchers now want to do more studies to identify which patients might benefit from the treatment and whether it should be used alongside other melanoma drugs that are already approved.
Drug regulators will be watching closely and will soon make a final decision about T-Vec.
Earlier this year an immunotherapy drug, pembrolizumab, became the first treatment "fast-tracked" for NHS patients in England with advanced melanoma, under a new government scheme.
Drugs approved through the Early Access to Medicines scheme, launched in England in April 2014, have been scrutinised by regulators weighing up the risks and benefits.
Melanoma is the sixth most common cancer in the UK and kills more than 2,000 people in Britain each year.
Damage to the skin by the sun's harmful UV rays increases your risk of developing this cancer.