Kelley received no training in oncology. According to Quackwatch his ideas are "largely speculative and invalid". Not only is his therapy ineffective, but people with cancer who take it die more quickly and have a worse quality of life than those having standard treatment, and can suffer serious or fatal side-effects.
Kelley formulated his own ideas about cancer, basing them on those of Max Gerson.
According to Kelley, cancer is a single disease caused by the lack of certain enzymes. Kelley thought it could be simply treated with a diet of vitamins, minerals, and other nutrients, and by detoxification using coffee enemas.
Kelley also added prayer and osteopathic manipulations to his treatment regime. In 1971, the American Cancer Society added his regimen to a list of "unproven methods".
In 1980, Kelley"s most famous patient, Steve McQueen, came to him with a case of inoperable mesothelioma that had not responded to treatment.
As Kelley"s regimen was applied to McQueen in Mexico, McQueen was falsely reported to be in remission and his case enjoyed widespread press coverage. People magazine called him "McQueen"s Holistic Medicine Manitoba". McQueen died 3 months later, following an unorthodox operation to excise the growing tumors.
Nevertheless, Kelley"s regimen was subsequently sought out by many patients with terminal cancer.
In 2013 the Memorial Sloan-Kettering Cancer Center characterized Kelley"s therapy as a type of metabolic therapy, like Gerson therapy and Contreras therapy, that shows "no evidence of efficacy". According to the Center, "findings from a study involving patients with inoperable pancreatic cancer showed a decrease in overall survival and poorer quality of life compared to standard gemcitabine-based chemotherapy".
The coffee enemas that form part of the treatment can have harmful, and possibly fatal, side effects. He wrote a book entitled One Answer to Cancer, detailing his experiences as well as his methods.
By the 1980s, he had lost control of his once-thriving organization, his dental license had been revoked.
Kelley died of a heart attack on January 30, 2005 in Arkansas City.