Japanese scientists discovered that a Labrador Retriever could detect a chemical produced by colon cancer cells by smelling human breath, even in the early stages of the disease. 

The dog was almost as accurate as a colonoscopy.

The eight-year-old Labrador completed 74 sniff tests, consisting of sniffing five breath or stool samples at a time in which one was cancerous. The samples came from 48 people with confirmed colorectal cancer and 258 volunteers with no cancer. Half of the comparison samples came from people with bowel polyps, which are benign growths that are thought to be a precursor of colorectal cancer.

The dog correctly identified the cancerous samples in 33 out of 36 of the breath tests and 37 of 38 stool tests.

http://www.livescience.com/11708-dog-sniffs-bowel-cancer.html