There is growing interest in the possibility that dogs, because of their incredible sense of smell, might be able to ‘smell’ cancer. The hypothesis was first raised in 1989 when doctors described a case of a woman concerned about a mole that her dog would constantly sniff at and had tried to bite off, which turned out to be a malignant melanoma. There have been several other reports since then of dogs detecting cancers by constantly sniffing or nudging an area of their owner’s body. Tumours produce volatile organic compounds, which are released into urine, exhaled breath and sweat. Even in minute quantities these compounds are thought to have a distinct odour, particularly in the early stages of cancer when cells are dividing. There have been only a few studies with small numbers of patients, but the results suggest dogs could be trained to detect these compounds. Studies over the past decade have shown trained dogs could identify the urine of patients with bladder cancer almost three times more often than would be expected by chance alone, detect lung cancer in exhaled breath samples with very high accuracy (in two separate studies) and identify ovarian and colorectal cancers by smelling breath samples. Scientists are also using chemical analysis and nanotechnology to try to identify cancer biomarkers in breath, sweat and urine that could be used in blood tests or other tests to detect cancer. If they can identify the chemical changes responsible for the odour the dogs are picking up, it may be possible to develop computerised screening instruments with the same sensitivity dogs have.