Toward the primary prevention of cancer.

This is the threshold of an era when many of the most prevalent human cancers can, to a significant extent, be prevented through life-style changes or medical interventions. For lung cancer, the leading cause of cancer deaths in the United States, the major cause, cigarette smoking, is known and strategies for reducing smoking are slowly succeeding. Dietary changes can reduce the risk of developing large bowel cancer, the second most common cancer overall. The etiology of the major cancer in women, cancer of the breast, is sufficiently well understood that large-scale medical intervention trials are imminent. Recent changes in the incidence and mortality of these and the other major human cancers are reviewed with a brief explanation as to why these changes have occurred, followed by a summary of the state of knowledge regarding the major causes of cancer.