After a study of the 176 cases (Everson and Cole) of spontaneous regression of cancer and recent progress in immunology, the author is convinced stimulation of the immune process is the most important factor in S.R. of cancer. Stimulating factors are numerous including bacterial products, enzymes, infections, hormones, trauma, etc. Of the 176 cases reported by Everson and Cole, 71 (40%) were associated with some type of operative trauma. Since the effective anticancer agent interferon is an important protective agent (especially antiviral) in the human body, this product could readily play an important role. Immunoglobulins appear to be possible factors. The blocking and unblocking agents of the Hellströms and associates support this supposition. Elimination of carcinogens appears important, considering the remarkable disappearance of cancer of the bladder in 12 of 13 patients having diversion of the urine from the bladder to the colon by transplantation of the ureters from the bladder to the colon. Innumerable antigens unknown to us at the present time could act as stimulants to our immune system, and thus cause regression of cancer. Hormonal factors must obviously be considered, but the author is doubtful that they exert an important role.